Absorbent members and articles containing superabsorbent polymer foam

ABSTRACT

An improved superabsorbent polymer foam having a morphology to provide improved absorptive properties is disclosed. The foam preferably comprises a superabsorbent polymer formed from a substantially water-soluble, unsaturated monomer having neutralized carboxyl groups and a substantially water-soluble internal crosslinking agent. The monomer and crosslinking agent are expanded in the presence of a substantially water-insoluble blowing agent and a suitable solvent and reacted to form a superabsorbent polymer foam having substantially continuous, intercommunicating channels substantially throughout the foam and a relatively high surface area to mass ratio. 
     Methods for making the foam and absorbent products, members and articles containing the foam are also disclosed.

This is a divisional of application Ser. No. 08/038,580, filed on Mar.26, 1993, now U.S. Pat. No. 5,338,766.

BACKGROUND OF THE INVENTION

A) FIELD OF THE INVENTION

The present invention relates to superabsorbent polymer foams and, moreparticularly, to an improved superabsorbent polymer foam having amorphology which provides improved absorptive properties, e.g.,substantially continuous intercommunicating channels substantiallythroughout the foam and a relatively high surface area to mass ratio.The superabsorbent polymer foams of the present invention are especiallyuseful in absorbent members which can be incorporated into absorbentarticles such as diapers, adult incontinence pads, sanitary napkins, andthe like. The present invention also relates to methods for producingsuch a superabsorbent polymer foam.

B) BACKGROUND INFORMATION

"Superabsorbent" polymer materials (also known as hydrogels,hydrocolloids, osmetics and absorbent gelling materials) are generallycapable Of absorbing large quantities of fluids such as water and bodyexudates and are further capable of retaining such absorbed fluids undermoderate pressures. These absorptive characteristics make such materialsespecially useful for incorporation into absorbent articles such asdiapers, sanitary napkins, and the like.

The art teaches superabsorbent foams formed of superabsorbent polymermaterial. For example, U.S. Pat. No. 4,529,739 issued to Scott et al. onJul. 16, 1985 and U.S. Pat. No. 4,649,164 issued to Scott et al. on Mar.10, 1987, teach a foamed, water-swellable, polymeric water absorbentmaterial prepared by contacting a polymer capable of having awater-swellable character and containing acid moleties in acid form witha blowing agent capable of neutralizing the acid moleties. U.S. Pat. No.4,808,637 issued to Boardman et al. on Feb. 28, 1989 and European PatentApplication No. 0295438 published by Boardman on Dec. 21, 1988, teach anacrylate superabsorbent composition having an improved rate ofabsorbency, low residual acid content and a low acrylate monomercontent, formed by uniformly reacting a mixture of acrylic acid, analkali metal salt of carbonic acid, aluminum acetate, sodium persulfateand water using a microwave heat source. U.S. Pat. No. 5,154,713 issuedto Lind on Oct. 13, 1992, teaches a superabsorbent polymer having anincreased rate of water absorption obtained by the addition, preferablyprior to polymerization, of a carbonate blowing agent to a monomersolution of the monomers used to form the superabsorbent polymer.

The art also teaches foams containing particulate, non-foamedsuperabsorbent materials. See, for example, U.S. Pat. No. 4,394,930issued to Korpman on Jul. 26, 1983; U.S. Pat. No. 4,415,388 issued toKorpman on Nov. 15, 1983; and Great Britain Patent Application 2136813Apublished by Korpman on Sep. 26, 1984. These references teach foamproducts prepared from solid, particulate superabsorbent polymer, ablowing agent, and a liquid polyhydroxy compound. U.S. Pat. No.4,725,629 issued to Garvey et al. on Feb. 16, 1988, teaches asuperabsorbent polyurethane foam based on an interpenetrating polymernetwork of a crosslinked polyurethane and crosslinked addition polymer,prepared by forming the polyurethane foam in the presence of additionpolymerizable monomers, a crosslinking agent and a free radicalinitiator.

While the above foams may be useful absorbent materials, they have notshown to be optimal for use in disposable products because theirabsorptive properties tend to be limited. The superabsorbent foamsformed of superabsorbent material tend to be characterized bydiscontinuous channels. Such foams tend to possess a relatively largeaverage cell size and wide cell size distribution, i.e., the foams havea relatively large diameter capillary structure in which the capillarydiameter varies widely and randomly. It is believed that the largeaverage cell size and discontinuous channels result in a foam producthaving a relatively low surface area to mass ratio such that the osmoticabsorptive rate is not optimal. In addition, it is believed that thediscontinuity of foam channels and a wide cell size distribution limitthe ability of fluids to flow by capillary transport through the foamstructure. This limitation of the capillary transport properties isbelieved to limit both the osmotic and capillary absorptive propertiesof such foams, more particularly the capillary absorptive capacity andrate and the osmotic absorptive rate. Thus, the absorptive rates andcapacities of such foam products tend to be limited.

In a composition in which particulate superabsorbent polymer materialsare present in a foam made of non-superabsorbent material such aspolyurethane, the foamed portion of the composition is notsuperabsorbent as defined herein since it does not have sufficientability to retain absorbed fluids. Therefore, although thesuperabsorbent particulate portion of the structure may be able toretain fluids, the overall capacity of the foam to absorb and retainfluids is limited. Further, the overall absorptive properties of thefoam tend to be limited due to the relatively low surface area to massratio of the particulate portion relative to the foam portion. This isbelieved to be particularly important for absorption of body fluidscontaining high molecular weight components, e.g., blood and menses.Such components are believed to deactivate particulate superabsorbentmaterials due to their large molecular size relative to the particulatematerial. The high molecular weight components may also deactivateregions of a foam characterized by a discontinuous channel. In addition,the use of polyurethane materials in applications intended for humancontact such as diapers presents concerns over the potential toxicity ofmaterials that may be used to prepare the polyurethane.

Thus, there is a continuing need in the field of superabsorbentmaterials to provide materials having faster absorptive rates andgreater absorptive capacities. More particularly, there is a need toprovide absorbent articles having such improved absorptive propertieswhile reducing the overall thickness of the absorbent article. Forabsorbent articles, it is desirable to use materials which are non-toxicto humans. Additionally, it is desirable to provide such materials andabsorbent articles in a cost-effective manner, e.g., by rapid, simpleand safe manufacturing techniques.

It is therefore an object of the present invention to provide asuperabsorbent polymer foam having an improved absorptive rate andimproved fluid distribution properties. An additional object of thepresent invention is to provide a superabsorbent polymer foam having amorphology which provides an increased absorptive rate, particularlycharacterized by a high surface area to mass ratio. Another object ofthe present invention is to provide such a superabsorbent polymer foamwhich does not present undesirable risks to human health. In addition,it is an object of the present invention to provide a cost-effectivemethod of making such foams.

An additional object of the present invention is to provide absorbentproducts, members and articles having improved absorptive rates. Anotherobject of the present invention is to provide absorbent members whoseabsorptive properties are tailored to the requirements of their intendedapplication. An additional object of the present invention is tominimize the thickness of absorbent articles while providing improvedabsorptive properties.

SUMMARY OF THE INVENTION

The present invention relates to a superabsorbent polymer foamcomprising superabsorbent polymer material. In a preferred embodiment,the superabsorbent polymer material of the foam is formed from asubstantially water-soluble, unsaturated monomer comprising neutralizedcarboxyl groups reacted with a substantially water-soluble internalcrosslinking agent. In a preferred embodiment, the monomer and internalcrosslinking agent are expanded in the presence of a blowing agent and asolvent so as to form an expanded structure, the solvent being a solventfor the monomer and internal crosslinking agent but not for the blowingagent or the superabsorbent polymer material (preferably water). Theexpanded structure is subjected to conditions so as to react the monomerand internal crosslinking agent to form the superabsorbent polymermaterial. The expansion and reaction are controlled such that theresultant superabsorbent polymer foam has a morphology which provides animproved absorptive rate, which may be characterized by substantiallycontinuous intercommunicating channels substantially throughout the foamand a relatively high surface area to mass ratio, small average cellsize, and low density.

It is believed that the superabsorbent polymer foams of the presentinvention have improved absorptive rates and capacities relative toknown superabsorbent foams. It has been found that when a foam of thepresent invention is contacted with fluids, the foam swells generallyisotropically even under moderate confining pressures, absorbs andtransports such fluids into and within the intercommunicating channels,and imbibes such fluids into the superabsorbent polymer material of thefoam, where they are retained. Since the foam has substantiallycontinuous intercommunicating channels substantially throughout the foamstructure, it tends to rapidly absorb fluids by substantiallyunrestricted capillary transport, particularly in the verticaldirection. In addition, since the intercommunicating channels allowdistribution of fluids throughout the foam, they provide increasedutilization of the fluid-retentive (osmotic absorptive) properties ofthe polymer making up the foam. Once saturated to its osmotic absorptivecapacity, it is believed that the intercommunicating channels of thefoams herein provide improved capillary absorptive capacity. The foamsof the present invention have a relatively high surface area to massratio which enhances the osmotic absorptive rate of the polymermaterial. Thus, the overall absorptive rate and capacity of the foamtends to be maximized.

The morphology of the foams of the present invention may also minimizedeactivation of the superabsorbent material of the foam by highmolecular weight fluids. In addition, the present foams tend to possessgreater flexibility as compared to known foams formed of superabsorbentpolymer material. Additionally, the superabsorbent polymer foams of thepresent invention do not present the undesirable risks to human healththat may be present in some known superabsorbent foams.

The invention also relates to improved absorbent products, absorbentmembers and absorbent articles incorporating the superabsorbent polymerfoams of the present invention. It is believed that the foams enhancethe fluid handling characteristics of such items by rapidly acquiringfluids and efficiently distributing and storing such fluids, therebyallowing for the acquisition and transport of subsequent loadings offluids. As a result, the overall absorptive rate and capacity of suchproducts, members and articles is increased.

The present invention also relates to methods of making thesuperabsorbent polymer foams. In a preferred embodiment, the foam isprepared by:

(I) forming a reaction mixture comprising the monomer, internalcrosslinking agent, and solvent;

(II) dispersing a blowing agent in the reaction mixture;

(III) stabilizing the dispersion of the blowing agent in the reactionmixture;

(IV) expanding the blowing agent;

(V) reacting the monomer and internal crosslinking agent so as to form asuperabsorbent polymer material; and

(VI) controlling the dispersion, stabilization, expansion, and reactionsteps such that the resultant superabsorbent polymer foam has amorphology which provides an improved absorptive rate and fluiddistribution properties.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the present invention, it is believed that thepresent invention will be better understood from the followingdescription in conjunction with the accompanying drawings in which:

FIG. 1 is a photomicrograph showing a top view of an edge of asuperabsorbent polymer foam of the present invention enlargedapproximately 50 times and also a portion of this view enlargedapproximately 250 times.

FIG. 2 is a photomicrograph showing a top view of an edge of thesuperabsorbent polymer foam in FIG. 1 taken from a plane perpendicularto the plane of the edge in FIG. 1, enlarged approximately 50 times, andalso a portion of this view enlarged approximately 250 times.

FIG. 3 is a plan view of a disposable diaper embodiment of the presentinvention having portions cut-away to reveal underlying structure, theouter surface of the diaper facing the viewer.

FIG. 4 is a partially cut-away plan view of a sanitary napkin embodimentof the present invention.

FIG. 5 is a graph showing a relationship of viscosity and temperaturefor a reaction mixture of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The superabsorbent polymer foams of the present invention are capable ofabsorbing large quantities of fluids (i.e., liquids) such as waterand/or body exudates (e.g., urine or menses) and are further capable ofretaining such fluids under moderate pressures, such as thoseencountered in the wearing of absorbent articles. The foams are formedof superabsorbent polymer material, commonly referred to as hydrogel,hydrocolloid, osmotic and absorbent gelling material. As used herein,"superabsorbent polymer" means a substantially water-insoluble, slightlycrosslinked, partially neutralized, hydrogel-forming polymer materialwhich is capable of absorbing and retaining large quantities of fluidssuch as water and/or body exudates. Superabsorbent polymer materialstypically have the ability to absorb and retain, under moderatepressures, such fluids in an amount of at least about 10 times,preferably at least about 15 times, most preferably at least about 50times, the dry weight of the polymer material. By "polymer foam" ismeant the structure which results when a relatively monomer-free gas orrelatively monomer-free liquid is dispersed as bubbles in a liquidcontaining polymerizable, superabsorbent-polymer-forming reactants,followed by expansion of the bubbles and polymerization of the reactantsin the liquid which surrounds the expanded bubbles. The resultantpolymerized, expanded structure can be in the form of a poroussolidified structure which is an aggregate of cells, the boundaries orwalls of which cells comprise solid polymerized material. Afterexpansion and polymerization, the cells themselves are typically free ofthe relatively monomer free gas or liquid which, prior to expansion andpolymerization, had formed the bubbles in the liquid dispersion.

In its broadest aspect, the superabsorbent polymer foams of the presentinvention comprise a superabsorbent polymer material formed from areaction mixture comprising (i) superabsorbent-polymer-forming reactantswhich are capable of being made substantially soluble in a solvent and(ii) such solvent. A blowing agent is dispersed in the reaction mixture,stabilized, and expanded so as to form an expanded structure. During orafter formation of the expanded structure, thesuperabsorbent-polymer-forming reactants are reacted so as to form apolymer material which is substantially insoluble in the solvent. (Thepolymer material may or may not be superabsorbent by the time thereaction has advanced to the point of insoluble polymer formation. Ifnot superabsorbent at that point, the polymer is further reacted inorder to render it superabsorbent.) The dispersion, stabilization,expansion, and reaction steps are controlled such that the resultantpolymer foam has a morphology to provide improved absorptive properties,which morphology may be characterized by substantially continuousintercommunicating channels substantially throughout the foam and arelatively high surface area to mass ratio.

A "dispersion of the blowing agent" in the reaction mixture means thatthe blowing agent is present in the form of discrete particles in thereaction mixture, the blowing agent particles being relatively free ofsuperabsorbent-polymer-forming reactants and solvent. By "substantiallysoluble" it is meant that the superabsorbent-polymer-forming reactant(or other component so described) may be dissolved or dispersed in thesolvent such that a minimum defined level of the reactant (or component)is extractable from the solvent. Substantially soluble reactants orcomponents are those which can be dissolved or dispersed in the solventsuch that at least about 50%, preferably at least about 60%, morepreferably at least about 75%, most preferably at least about 80%, ofthe reactant or component which was dissolved or dispersed in thesolvent is extractable. "Substantially insoluble" means that a reactant,component, reaction product, or other material (including thesuperabsorbent polymer material of the superabsorbent polymer foam) sodescribed can be dissolved or dispersed in the solvent such that lessthan about 50%, preferably less than about 40%, more preferably lessthan about 25%, most preferably less than about 20%, of such reactant,component, reaction product or other material which was dissolved ordispersed in the solvent is extractable. A suitable method fordetermining extractable levels is described in the TEST METHODS section.Typically, the substantially insoluble polymer materials formed hereinpossess a relatively high weight average molecular weight, e.g., of atleast about 200,000 grams/mole. Such a high molecular weight is atypical characteristic of network crosslinked polymers having a certaindegree of network crosslinking. In contrast, "substantially solublepolymers" are typically substantially linear, i.e., the polymer moleculeis generally two-dimensional with no or only a minimal degree ofcrosslinking.

In a preferred embodiment, the superabsorbent polymer foam comprises asuperabsorbent polymer material formed from:

(I) a substantially water-soluble, unsaturated monomer comprisingneutralized carboxyl groups; and

(II) a substantially water-soluble internal crosslinking agent reactedwith the monomer.

The preferred superabsorbent polymer foam is preferably formed by firstforming a reaction mixture comprising the monomer, the internalcrosslinking agent, and a suitable solvent (preferably water). A blowingagent is then stably dispersed in the reaction mixture, followed byexpansion of the blowing agent and reaction of the monomer and theinternal crosslinking agent so as to form a foam comprised of asubstantially water-insoluble polymer material. (Depending on thereactant type and conditions, the insoluble polymer material may or maynot be superabsorbent and may therefore require further reaction, e.g.,neutralization of carboxyl groups and/or further crosslinking of thepolymer material to render it superabsorbent.) The dispersion,stabilization, expansion, and reaction steps are controlled such thatthe foam has a morphology which may be characterized byintercommunicating channels and a relatively high surface area to massratio.

Formation of the Reaction Mixture

One component of the preferred reaction mixture is a substantiallywater-soluble monomer comprising neutralized or neutralizable carboxylgroups. The monomer preferably contains sufficient carboxyl groups suchthat a linear polymer thereof is substantially water-soluble (i.e., thecarboxyl groups are hydrophilic). Mixtures of such monomers may also beused.

The monomers comprising carboxyl groups include acid, acid anhydride,and ester group containing monomers. These monomers may also containother hydrophilic groups, such as hydroxyl groups, amide-groups, aminogroups, nitrile groups, and quaternary ammonium salt groups. Preferably,the monomer contains acid type hydrophilic groups. More preferably, themonomer contains at least about 5 mole percent, most preferably at leastabout 10 mole percent, of acid groups.

Monomers containing carboxyl groups include the olefinically unsaturatedacids, esters thereof, and anhydrides which contain at least one carbonto carbon olefinic double bond. More specifically, these monomers can beselected from olefinically unsaturated carboxylic acids, esters of suchcarboxylic acids, acid anhydrides, sulfonic acids, esters of suchsulfonic acids, and mixtures of any two or more of the foregoingmonomers.

Olefinically unsaturated carboxylic acid and carboxylic acid anhydridemonomers include the acrylic acids and derivatives thereof, typified byacrylic acid itself, methacrylic acid, ethacrylic acid,alpha-chloroacrylic acid, alpha-cyano acrylic acid, beta-methyl acrylicacid (i.e., crotonic acid), alpha-phenyl acrylic acid, beta-acryloxypropionic acid, and beta-steryl acrylic acid; maleic acid; and maleicacid anhydride. Other monomers of this type are sorbic acid,alpha-chloro sorbic acid, angelic acid, cinnamic acid, p-chloro cinnamicacid, itaconic acid, citraconic acid, mesaconic acid, glutaconic acid,aconitic acid, fumaric acid, and tricarboxyethylene.

Olefinically unsaturated sulfonic acid monomers and derivatives thereofinclude aliphatic or aromatic vinyl sulfonic acids such as vinylsulfonicacid, allyl sulfonic acid, vinyltoluene sulfonic acid and styrenesulfonic acid; and acrylic and methacrylic sulfonic acid derivativessuch as sulfoethyl acrylate, sulfoethyl methacrylate, sulfopropylacrylate, sulfopropyl methacrylate, 2-hydroxy-3-acryloxy propyl sulfonicacid, 2-hydroxy-3-methacryloxy propyl sulfonic acid and2-acrylamido-2-methyl propane sulfonic acid.

The carboxyl groups (e.g., acid groups) are at least partiallyneutralized with cations capable of forming a salt with the monomer toform a monomer having neutralized carboxyl groups. Such salt-formingcations include, for example, alkali or alkaline metals, ammonium,substituted ammonium and amines as discussed in further detail in U.S.Pat. No. Re. 32,649, Brandt et al., Apr. 19, 1988, incorporated hereinby reference. Neutralization is preferably carried out in anyconventional manner which results in at least about 25 mole percent,more preferably at least about 50 mole percent, most preferably at leastabout 75 mole percent, of the total carboxyl groups being neutralized.The carboxyl groups are preferably neutralized prior to formation of thesubstantially water-insoluble polymer foam, e.g., neutralization ispreferably carried out on the monomer or of a water-soluble polymerthereof.

Monomers possessing hydrophilic groups other than carboxyl groups may beused with the carboxyl group containing monomer. Other hydrophilicgroups include hydroxyl groups, amide-groups, amino groups, nitrilegroups, and quaternary ammonium salt groups. Monomers containing suchgroups are well known materials and are described in greater detail, forexample, in U.S. Pat. No. 4,076,663 issued to Masuda et al. on Feb. 28,1978; and U.S. Pat. No. 4,062,817 issued to Westerman on Dec. 13, 1977;which patents are incorporated herein by reference. One or more types ofsuch hydrophilic groups may be present in the monomer.

Although this disclosure is generally in terms of the monomer, it is tobe understood that substantially water-soluble homopolymers, copolymers,or reaction products of the monomer may also be used in place of or inaddition to the toohomer form. Such alternative starting materialsinclude substantially water-soluble homopolymers of the toohomer andsubstantially water-soluble reaction products of the monomer or itshomopolymer and the internal crosslinking agent. For example, asubstantially linear, substantially water-soluble homopolymer can beformed by subjecting the monomer to known polymerization conditions. Asubstantially water-soluble, partially crosslinked polymer may also beformed by reacting (e.g., by heating) the monomer or linear polymerthereof with a crosslinking agent such as the internal crosslinkingagents herein. Such a polymer would typically have a low level ofcrosslinking, e.g., less than about 5%.

The above polymers of the monomer (especially a linear polymer of themonomer) may be preferred starting materials in order to control theviscosity of the reaction mixture so as to aid formation of a foamhaving a desired morphology, particularly a relatively small cell size.In general, a reaction mixture using such polymers will have a higherviscosity compared to a reaction mixture starting with the toohomeritself. Use of such polymers may also be particularly useful where foamshaving a low residual monomer content are desired, since residualmonomer which may be present in the polymer of the monomer (e.g., alinear polymer of the monomer) may be removed by known processes, e.g.,flashing, prior to use in the reaction mixture.

The preferred reaction mixture additionally comprises a substantiallywater-soluble internal crosslinking agent. Suitable internalcrosslinking agents are compounds which are capable of reacting with themonomer to form a substantially water-insoluble, preferablysuperabsorbent, polymer material. Preferably, the internal crosslinkingagent will be such that the reaction mixture has an activationtemperature which is greater than the vaporization temperature of theblowing agent and the critical temperature. The activation temperatureis the temperature, for a given reaction mixture, at which the internalcrosslinking agent will react with the monomer to form a high molecularweight, network crosslinked polymer. The activation temperature can bedetermined by differential scanning calorimetry (i.e., DSC). DSCtechniques are generally described in Analytical Calorimetry, Vol. 3,Roger S. Porter et al., Plenum Press (1974), pages 17-44, incorporatedherein by reference. Suitable DSC equipment includes the Perkin-ElmerDSC 7 Series Thermal Analysis System, available from Perkin-Elmer ofNorwalk, Conn. A suitable method for determining the activationtemperature is described in the manual Perkin-Elmer 7 Series ThermalAnalysis System, Perkin-Elmer, Jan. 1990, incorporated herein byreference.

Suitable internal crosslinking agents include compounds having at leasttwo polymerizable double bonds; compounds having at least onepolymerizable double bond and at least one functional group reactivewith the monomer; compounds having at least two functional groupsreactive with the monomer; and polyvalent metal compounds which can formionic linkages. One or more internal crosslinking agents may be used.Typical internal crosslinking agents are described in greater detail inthe above cited U.S. Pat. No. 4,076,663. Selection of a particularinternal crosslinking agent is determined in part by the desired rate ofreaction of the internal crosslinking agent with the monomer at aparticular temperature, which rate can be readily determined by onehaving ordinary skill in the art. The internal crosslinking agent ispreferably selected from the group consisting ofN,N'-methylenebisacrylamide, triallylamine, triallylphosphate, and di-or poly- glycidyl ethers of aliphatic polyvalent alcohols. Mostpreferably, N,N'-methylenebisacrylamide is used.

The internal crosslinking agent will typically be present in thereaction mixture in an amount of from about 0.001 mole percent to about5 mole percent based on the total moles of monomer present in themixture (i.e., about 0.01 to about 20 parts by weight, based on 100parts by weight of the monomer material).

The reaction mixture also comprises a solvent. Suitable solvents includeany in which the monomer and internal crosslinking agent aresubstantially soluble and in which the superabsorbent polymer andblowing agent are substantially insoluble. Suitable solvents will alsohave a vaporization temperature which is greater than the vaporizationtemperature of the blowing agent and the critical temperature of thereaction mixture, and preferably greater than the activationtemperature. In the preferred embodiment where substantiallywater-soluble monomers are used, the solvent is preferably water, awater-soluble alcohol (e.g., lower alcohols such as methanol, ethanol,propanols, and butanols), or mixtures of any two or more of suchcompounds. Most preferably, the solvent is water. Although theconcentrations of the various components of the reactant mixture may bewidely varied as long as the dispersion, stabilization, expansion, andreaction steps can be effected, the reaction mixture will generallycomprise about 1 part by weight of the monomer to about 1 part by weightof solvent.

The reaction mixture may also contain various optional components,including surfactants, polymerization initiators, and viscosity controlagents. Conventional additives for absorbent gelling materials, such asantioxidants and deodorants, may also be included.

It is particularly desirable to include at least one surfactant in thereaction mixture. The surfactant may be any of those generally known inthe foam art, selected for its ability to stabilize the dispersion ofthe blowing agent in the reaction mixture. The surfactant may aid in theformation of a superabsorbent polymer foam having a desired morphology.The surfactant is believed to be particularly useful for obtainingrelatively small average cell sizes and relatively narrow cell sizedistributions by influencing the particle size and particle sizedistribution of the blowing agent.

Where the solvent is water or water-soluble and the blowing agent issubstantially water-insoluble, suitable surfactants include nonionicsurfactants possessing both hydrophilic and hydrophobic groups. One ormore surfactants to provide such groups may be used. The presence ofboth hydrophilic groups and hydrophobic groups may serve to enhancestabilization by preventing phase separation into an "oily" blowingagent phase and a "water" reactant phase. A mixture of such groupsallows for tailoring of the surfactant system to enhance stabilizationof the dispersion, depending on the respective hydrophilicity orhydrophobicity of the components of the reaction mixture. In general, asthe ratio of hydrophobic groups to hydrophilic groups in the surfactantincreases, stabilization is further enhanced, provided that sufficienthydrophilic groups to prevent phase separation are present. Preferably,the surfactant (or surfactant mixture) is selected to match thehydrophile-lipophile balance (i.e., HLB) value of the blowing agent. Forsubstantially water-insoluble blowing agents, the blowing agent is morereadily stabilized as the HLB value of the surfactant decreases. Asystem of matching the HLB value of an emulsifier to one or moreingredients to be emulsified is described in the technical bulletinMeaning of HLB Advantages and Limitations, ICI Americas Inc., 1984,incorporated herein by reference.

The nonionic surfactant is preferably selected from the group consistingof linear alkoxylated alcohols, linear alkylphenoxylated alcohols, andesters thereof. Preferred are the ethoxylated copolymers of sorbitanfatty acid esters such as the ethoxylated copolymers of sorbitanmonolaurate (available as TWEEN®20, ICI Americas, Inc.) and of sorbitanmonooleate (available as TWEEN®80, ICI Americas, Inc.). Mixtures of theforementioned surfactants with substantially hydrophobic (i.e.,lipophilic) surfactants, may also be used. Suitable hydrophobicsurfactants include sorbitan fatty acid esters such as sorbitanmonolaurate (available as SPAN®20, ICI Americas, Inc.) and sorbitanmonooleate (available as SPAN®80, ICI Americas, Inc.).

While the amount of surfactant may be varied, it is generally desirableto use the minimum level needed to effect stabilization of the blowingagent. Use of the minimum level may prevent or minimize an adverseimpact on the absorptive properties of the resultant foam due toresidual surfactant and generally provides a product cost benefit. Thesurfactant will typically be used in an amount of about one-tenth theamount of the blowing agent. The surfactant can be added to the reactionmixture or pre-mixed with the blowing agent which is later dispersed inthe reaction mixture.

The optional initiator is particularly useful to initiatepolymerization, generally of the monomer and the internal crosslinkingagent. An initiator is usually preferred since it typically lowers theactivation temperature of the internal crosslinking agent. An initiatoris particularly suitable where an increased rate of reaction is desired,and is therefore generally preferred for cost-effective manufacturing.In addition, an initiator is particularly useful where the foam isformed on a wicking substrate. Upon activating the initiator and therebyinitiating polymerization, the viscosity of the dispersion typicallyincreases. The increase in viscosity may serve to facilitate theformation of a desired foam pattern on the wicking substrate.

The initiator may be any conventional polymerization initiator. Theinitiator is preferably substantially soluble in the solvent and isselected based, in part, on the intended method of inducing the reactionof the monomer. Thus, light and heat activated initiators are usefulinitiators. For example, where thin films of the foams are desired, alight activated initiator may be used to effect rapid reaction when themixture is exposed to a light radiation source. The optionalpolymerization initiators include free radical initiators including, forexample, per oxygen compounds such as sodium, potassium, and ammoniumpersulfates, caprylyl peroxide, benzoyl peroxide, hydrogen peroxide,cumene hydroperoxides, tertiary butyl diperphthalate, tertiary butylperbenzoate, sodium peracetate, sodium percarbonate, and the like.Conventional redox initiator systems can also be used, e.g., systemscombining the foregoing peroxygen compounds with reducing agents such assodium bisulfite, L-ascorbic acid or ferrous salts. The amount of theinitiator used may be that amount conventionally used in the polymerfield. Typically, the initiator is used at a level of up to about 5 molepercent, preferably about 0.001 to 0.5 mole percent, based on the totalmoles of polymerizable monomer.

Another preferred, optional component of the reaction mixture is aviscosity control agent. The viscosity control agent generally serves toincrease the viscosity of the reaction mixture and is particularlyuseful for controlling the particle size of the blowing agent which isdispersed in the reaction mixture. In general, as the viscosity of thereaction mixture increases the particle size is more readily controlled.Thus, the viscosity control agent may be used to disperse and stabilizethe blowing agent in the reaction mixture. The viscosity control agentmay also facilitate coating or printing of the stable dispersion onto asubstrate, particularly a wicking substrate. By increasing the viscosityof the reaction mixture, the agent may facilitate the formation of adesired foam pattern on the substrate. The viscosity control agent mayalso prevent or minimize separation of the solvent from the reactionmixture (i.e., partitioning), particularly when the stable dispersion isapplied to a wicking substrate, thereby promoting the formation of auniform product having optimum properties. Suitable viscosity controlagents are preferably substantially soluble in the solvent. Exemplarywater-soluble viscosity control agents are carboxymethyl cellulose,hydroxyethyl cellulose, and polyacrylic acid. The viscosity controlagent is typically added in an amount of less than about 2%, moretypically less than about 1%, by weight of the total monomer.

Formation of a Stable Dispersion

A stable dispersion of a blowing agent is formed in the reaction mixtureby adding a blowing agent to the reaction mixture, typically eitherduring or after formation of the reaction mixture; dispersing theblowing agent; and stabilizing the dispersion. The blowing agent isdispersed in the reaction mixture and stabilized so as to form a stablediscontinuous phase of the blowing agent (i.e., "particles" of blowingagent) in the reaction mixture phase. The blowing agent particles arerelatively free of the monomer, internal crosslinking agent, andsolvent.

Suitable blowing agents include any conventional blowing agent which issubstantially insoluble in the solvent and whose particle size can becontrolled and stabilized when dispersed in the reaction mixture. Inaddition, the blowing agent will be capable of controlled expansion.Suitable blowing agents also have a vaporization temperature (i.e.,boiling point) which is less than the vaporization temperature of thesolvent, at a given pressure. The blowing agent will preferably alsohave a boiling point which is less than the critical temperature, so asto allow sufficient expansion of the blowing agent before the formationof the substantially water-insoluble polymer. Exemplary blowing agentsare disclosed in Chemical Encyclopedia, H. Lasman, NationalPolychemicals, Inc., Vol. 2 at page 534, incorporated herein byreference. In the preferred embodiment in which water is used as thesolvent, suitable blowing agents are substantially water-insolubleliquids having a boiling point of less than about 100° C., morepreferably less than about 80° C., most preferably less than about 50°C. Typically, the blowing agent will have a boiling point in the rangeof about -20° C. to about 80° C., preferably about -20° C. to about 50°C. Such blowing agents include aliphatic and aromatic hydrocarbons andhalohydrocarbons, which may be cyclic or alicyclic, linear or branched,and saturated or unsaturated. Exemplary blowing agents include thepentanes, hexanes, heptanes, benzene, substituted benzenes,chloromethanes, chloroethanes, chlorofluoromethanes, andchlorofluoroethanes such as described in the above referenced ChemicalEncyclopedia. Preferably a pentane (e.g., n-pentane, 2-methylbutane,and/or 2,2-dimethylpropane) or 1,1,2-trichlorotrifluoroethane is used.While the amount of blowing agent employed can vary over a wide rangeconsistent with obtaining a foam having a desired morphology, theblowing agent will generally be added at a level of about 5 to about 50parts per 100 parts (by weight) of monomer. Typically, about 20 to about30 parts of blowing agent per 100 parts (by weight) of monomer are used.

The blowing agent may be dispersed by applying shear stress (e.g.,through high shear mixing) to the reaction mixture and, if necessary, bycontrolling the viscosity ratio of the blowing agent phase to thereaction mixture phase (as used herein, the viscosity ratio refers tothe viscosity of the blowing agent phase divided by the viscosity of thereaction mixture phase) and/or by using a surfactant. The dispersionprocess is controlled so as to obtain a desired blowing agent particlesize. The particle size of the dispersed blowing agent influences thecell size (including cell size distribution), the intercommunication offoam channels, and the surface area to mass ratio of the resultantsuperabsorbent polymer foam. Particle size influencing features includethe shear rate, surfactant type, the viscosity ratio, and the isotropyof the reaction mixture. Preferably, these features are controlled so asto minimize the blowing agent particle size. The blowing agent istypically dispersed to a particle size of less than about 10 microns,preferably less than about 5 microns, more preferably less than about 2microns. The minimum particle size is typically about 0.1 microns.

For obtaining a relatively small blowing agent particle size, it ispreferred to use a relatively high shear stress for dispersing theblowing agent. In general, the higher the rate of shear, the smaller theaverage particle size of the blowing agent. Where particles ofsubstantially uniform size are desired, it is typically preferred tohave uniform shear throughout the mixture.

For a given reaction mixture, blowing agent, temperature, and shearstress, the particle size of the blowing agent typically decreases asthe viscosity ratio of the dispersed blowing agent phase to thecontinuous reaction mixture phase is decreased. As the viscosity ratiodecreases, the blowing agent particle size is more readily controlled toa smaller particle size. Therefore, it is generally preferred tominimize the viscosity ratio. Typically, the dispersion is such that theviscosity ratio is less than about 0.5, more preferably less than about0.25.

The viscosity ratio is preferably decreased by using a viscosity controlagent in the continuous reaction mixture phase and/or a low viscosityblowing agent. The viscosity ratio may also be varied by varying thereaction conditions, for example, the temperature or reactantconcentration of the reaction mixture ("reactant" refers to thesuperabsorbent-polymer-forming materials, including the monomer andinternal crosslinking agent). In general, as the temperature decreasesor the reactant concentration increases, the viscosity ratio willdecrease. The viscosity ratio will also vary with the reactivity of thespecific superabsorbent-polymer-forming materials that are present inthe reaction mixture. The reaction conditions and reactant types may bevaried to control the rate of reaction in solution and therefore thesolution viscosity. In general, as the reaction progresses towardformation of the substantially water-insoluble polymer material (e.g.,as a linear polymer of the monomer is formed), the solution viscositywill increase and the viscosity ratio will decrease. In addition, theviscosity ratio may be decreased by initially using a substantiallywater-soluble polymer of the monomer as a superabsorbent-polymer-formingmaterial in the reaction mixture. Where a relatively uniform particlesize is desired, it is preferred that the reaction mixture besubstantially isotropic, i.e., the components of the reaction mixtureare blended such that they are substantially uniform throughout themixture.

The dispersion having the desired blowing agent particle size ispreferably stabilized prior to the expansion and reaction steps to formthe substantially water-insoluble polymer foam. Preferably,stabilization occurs simultaneously with dispersion. By "stable","stabilized", etc., it is meant that the desired particle size of thedispersed blowing agent is maintained for a time sufficient to allowreaction of the monomer and internal crosslinking agent to form thesubstantially water-insoluble polymer foam having a desired morphology,e.g., substantially continuous intercommunicating channels substantiallythroughout the foam and a relatively small cell size, low density, andhigh surface area to mass ratio.

Any method of stabilizing the dispersion may be employed. Preferably, asurfactant is used to stabilize the dispersion. In general, the smallerand more uniform the blowing agent particles, the more stable thedispersion. Therefore, stabilization may also be aided by controllingthe viscosity ratio. In general, the lower the viscosity ratio at agiven shear, the smaller the particle size of the blowing agent and themore stable the dispersion.

Expansion and Reaction

After forming the stable dispersion of the blowing agent having thedesired particle size, the blowing agent is expanded to form an expandedstructure, and the monomer or the monomer and the internal crosslinkingagent are reacted to form a substantially solvent-insoluble polymer(i.e., an expanded, substantially solvent-insoluble polymer structure isformed). In the preferred embodiment where the solvent is water, thepolymer is substantially water-insoluble.

The expansion and reaction are controlled such that, by the time thesubstantially water-insoluble polymer is formed, the expanded structurehas a morphology substantially as desired in the superabsorbent polymerfoam (the polymer need not be superabsorbent by :the point ofinsolubility). Preferably, the expansion of the blowing agent iscontrolled along with the reaction of the monomer or the monomer and theinternal crosslinking agent so as to provide a superabsorbent polymerfoam having substantially continuous intercommunicating channelssubstantially throughout the foam, an average cell size of less thanabout 100 microns, a surface area to mass ratio of at least about 0.2 m²/g, and a density of less than about 0.5 g/cm³.

In general, the blowing agent particles of the stabilized dispersion areexpanded so as to avoid excessive coalescence of the blowing agent as itexpands, i.e., the blowing agent particles generally expand in relativeproportion to their initial stabilized particle size and shape in thedispersion. Typically, the blowing agent particles are expanded to about10 times their original size. During or after expansion (preferablyafter), the monomer and or monomer and internal crosslinking agent arereacted to form the substantially water-insoluble polymer so as tostabilize the expanded structure (a substantially water-insolublepolymer foam is formed).

In general, the rate of expansion of the blowing agent and theviscoelastic properties of the reaction mixture as the substantiallywater-insoluble polymer forms are controlled simultaneously so as toprovide an open-celled foam having a desired morphology. If expansion istoo rapid or too slow relative to the formation of the substantiallywater-insoluble polymer, the foam may not have a desired morphology,particularly a desired surface area to mass ratio. The elasticity of thereaction mixture and/or forming polymer should be capable of supportingthe formation of the substantially water-insoluble polymer materialgenerally in a size and shape proportional to the expanded blowing agentparticles. In general, the elasticity of the reaction mixture and/orforming polymer should be capable of withstanding the vapor pressure ofthe expanding blowing agent.

For any given reaction mixture, there is a critical viscosity at which afoam having a desired morphology is difficult to achieve. The criticalviscosity corresponds to a critical temperature, described below. Thecritical viscosity is typically reached when the monomer or monomer andinternal crosslinking agent have polymerized such that the visco-elasticproperties of the resultant polymer make it difficult to form anopen-celled foam. Therefore, in preparing the foams herein, the blowingagent particles are preferably expanded to form an expanded structurebefore the critical viscosity of the reaction mixture is reached. Morepreferably, the expanded structure has substantially the finally desiredsuperabsorbent polymer foam morphology by the time the criticalviscosity is reached. Preferably, at least about 90%, more preferably atleast about 95%, of the ultimately desired open cells are formed beforethe point of critical viscosity.

At the point of critical viscosity, the polymer is typicallysubstantially water-insoluble. The substantially water-insoluble polymermay be formed by polymerization of the monomer alone. This reaction canbe initiated by heat and/or light radiation. The resultant polymer maybe further reacted to form a superabsorbent polymer material, e.g., byreacting the polymer with a crosslinking agent (and neutralizingcarboxyl groups of the polymer as may be necessary). In the preferredembodiment where an internal crosslinking agent is present in thereaction mixture, the polymer is crosslinked by heating the expanded,substantially water-insoluble polymer structure to a temperature greaterthan the critical temperature, generally to at least the activationtemperature. Alternatively or in addition to the internal crosslinkingagent, the polymer structure may be reacted with an externalcrosslinking agent.

The substantially water-insoluble polymer may also be formed by reactionof the monomer (or a substantially water-soluble polymer thereof) andthe internal crosslinking agent. This reaction is typically initiated byheat, either directly applied to the reactants or as generated by amonomer polymerization reaction induced by light radiation. Theresultant polymer may be further reacted as may be necessary to form asuperabsorbent polymer material, e.g., by subjecting the expandedpolymer structure to higher temperatures or by reacting the polymer ofthe structure with additional crosslinking agent (and neutralizingcarboxyl groups of the polymer as may be necessary).

Where the reaction to form a substantially water-insoluble polymer isinitiated by heat, there is a critical temperature at which the reactiontypically occurs. The critical temperature corresponds to the criticalviscosity and thus typically to the formation of a substantiallywater-insoluble polymer. The critical temperature and critical viscositywill vary with the particular reaction mixture composition, e.g., themenomer, internal crosslinking agent, and the optional initiator.

The critical temperature may be determined for a given reaction mixtureby determining the maximum rate of change in the viscosity of thereaction mixture as a function of temperature. The viscosity of thereaction mixture at various temperatures can be determined by knownrheometry methods. As used herein, viscosity refers to the apparentviscosity, i.e., viscosity is meant to characterize the full spectrum offluids or dispersions. This full spectrum includes simple Newtonianfluids for which viscosity is the proportionality constant that relatesshear force per unit area to the negative of the local velocitygradient, and other fluids that are not typically included by thissimple law (e.g., pastes, slurries, high molecular weight polymers anddispersions of insoluble materials mixed with soluble materials). Theseconcepts are described in greater detail in Transport Phenomenon, Bird,et al., John Wiley & Sons, Inc. (1966), Chpt. 1, incorporated herein byreference.

Since the critical temperature may vary with the reaction rate of agiven reaction mixture and therefore the time for which the reactionmixture is maintained at a given temperature, the viscosities aremeasured for a reaction mixture which has been maintained at varioustemperatures for a constant time period at each temperature. This ismost readily done by using a rheometer capable of heating a reactionmixture sample at a controlled rate. Suitable rheometers include aSangamo Visco-Elastic Analyzer and a Brabender blender and rheometer.The principle of operation of a blender and rheometer such as theBrabender system is described in the Handbook of Polymer Science andTechnology. Vol. 3: Applications and Processing Operation,Cheremisinoff, Marcel Dekker Inc. (1989), pages 373-419. Alternatively,the viscosity of separate samples of a given reaction mixture may bedetermined with the viscosity of the individual samples being measuredafter being held for a constant time period at a given temperature for agiven sample.

The viscosity is plotted as a function of temperature, the resultantcurve typically being as shown in FIG. 5. As shown in FIG. 5, theresultant curve has an initial, substantially linear, slightly slopingpart 5a corresponding to a small change in viscosity per unit change intemperature, and a second part 5b of rapidly increasing slopecorresponding to a rapid increase in viscosity per unit change intemperature. At least the first part 5c of this second part 5b may bedescribed as a parabolic, corresponding to an exponential increase inviscosity per unit change in temperature. This paraboloid part 5c isfollowed by a substantially linear, steeply sloping part 5d.

As shown in FIG. 5, the critical temperature (T_(c)) and criticalviscosity (V_(c)) are determined from the point of intersection of theline tangent to the substantially linear, slightly sloping part 5a andthe line tangent to the substantially linear, steeply sloping part 5d.The critical temperature and critical viscosity are determined byextrapolating from the point of intersection to the x-axis (T_(c)) andthe y-axis (V_(c)).

In some cases the parabolic part 5c may not be followed by asubstantially linear part but the curve may, for example, remainparabolic. In addition, for some systems it may be difficult to measurethe viscosity for all points on the parabolic curve (i.e., above acertain temperature). In such cases the critical temperature may bedetermined through calculus methods. The first derivative of the curve(viscosity as a function of temperature) is determined for varioustemperatures and is plotted as a function of those temperatures. Theresultant curve is typically characterized by a maximum inflection pointwhich corresponds to the maximum rate of change in the viscosity of thereaction mixture as a function of temperature. The temperature whichcorresponds to this maximum rate of change can be determined graphicallyand is the critical temperature.

In order to carry out the expansion step before the critical viscosityis reached, the blowing agent particles of the stable dispersion willtypically be expanded by heating the stable dispersion to a temperaturewhich is greater than or equal to the vaporization temperature of theblowing agent and less than or equal to the critical temperature,preferably less than the critical temperature, most preferably about 5°C. to about 10° C. less than the critical temperature. Therefore, theblowing agent is preferably selected such that it will volatilize(vaporize) at such a temperature. In an alternative embodiment of thepresent invention, the expansion is caused by decreasing the pressure onthe stable dispersion. In addition, various combinations of pressuresand temperatures may be selected in order to expand the dispersion.

Where a decrease in pressure alone is used to expand the blowing agent,the critical viscosity is not generally attained in the expansion step.In such case, it is important to form the substantially water-insolublepolymer at about the point where the expanded structure hassubstantially the same morphology as that desired in the final foam.This will typically be achieved by increasing the temperature of theexpanded structure to at least the critical temperature to causereaction of the monomer or the monomer and internal crosslinking agentto form the substantially water-insoluble polymer.

It is generally preferred to expand the blowing agent as slowly aspossible. Typically, the blowing agent is expanded by heating the stabledispersion to the vaporization temperature of the blowing agent at arate of less than about 1° C./minute, more preferably less than about0.5° C./minute, most preferably less than about 0.1 to about 0.2°C./minute. The rate of heating may be increased if a counterpressure isapplied to the dispersion in order to achieve substantially the samerate of expansion as where only the temperature is increased at thepreferred rates. Alternatively, where a decrease in pressure is used toexpand the blowing agent, a corresponding (at a given temperature)controlled rate of decreasing pressure may be used to form the expandedstructure.

As previously stated, the monomer or the monomer and internalcrosslinking agent of the expanded structure are reacted so as to forman expanded structure comprising a substantially water-insoluble polymer(i.e., a substantially water-insoluble foam is formed). Reaction can becaused by heating the expanded structure to at least the criticaltemperature. Preferably the expanded structure is heated to the criticaltemperature. The monomer and internal crosslinking agent are thenreacted to form a network crosslinked, typically superabsorbent,polymer. Such reaction is caused by heating the structure to at leastthe activation temperature. The activation temperature may be equal toor greater than the critical temperature. Preferably, the reactionmixture is designed (more particularly, the internal crosslinking agentand/or initiator is selected) such that the activation temperature isgreater than the critical temperature, more preferably at least about10° C. to about 20° C. above the critical temperature. Networkcrosslinking is described in greater detail below.

The rate of heating to cause reaction of the monomer (or polymerthereof) and the internal crosslinking agent to form the networkcrosslinked polymer is preferably slow in order to avoid the rapidvolatilization of any residual blowing agent and to prevent or minimizedisruption of the expanded structure. Where the activation temperatureis greater than the critical temperature, the rate of heating to causereaction need not be as slow as the rate of heating to cause expansion.Preferably, the rate of heating to cause reaction is less than about 10°C./minute, more preferably from about 5° C. about 10° C./minute, mostpreferably from about 8° C. to about 10° C./minute.

Although the various components of the stable dispersion may be selectedsuch that the blowing agent vaporization temperature, the criticaltemperature, and the activation temperature are the same, for thepreferred foams herein, those components are selected such that theblowing agent vaporization temperature is less than the criticaltemperature which itself is less than the activation temperature. Mostpreferably the components are such that the blowing agent vaporizationtemperature is from about 5° C. to about 10° C. less than the criticaltemperature which itself is from about 10° C. to about 20° C. less thanthe activation temperature.

The expansion and reaction conditions are preferably uniform throughoutthe dispersion and/or the expanded structure. Uniformity is generallyachieved by ensuring a relatively isotropic reaction mixture and auniform temperature and/or pressure throughout the dispersion and/or theexpanded structure. Uniformity of temperature and pressure areinfluenced by the heating means, the volume and/or depth of thedispersion being reacted, the heat flux (i.e., rate of heating), and theextent of exothermic heat evolution by the reacting dispersion.Preferably, these factors are Controlled so as to ensure uniform heattransfer (i.e., temperature gradients across the expanding material areminimized) throughout the dispersion and/or the expanded structure. Forforming a bulk foam material, a particularly suitable reactor forensuring uniform heat transfer is a stirred tube reactor. In formingthin films (e.g., having a thickness of from about 1 mil to about 500mils) of the foam, it will generally be suitable to use ultraviolet,infrared, microwave and/or electron beam radiation to cause expansionand/or reaction. For achieving a rapid rate of reaction in such thinfilms, it is generally preferred to use ultraviolet radiation. However,as the thickness of the foam to be formed increases over about 500 mils,it is preferred to use infrared and/or electron beam radiation in orderto prevent or minimize nonuniform cell structure.

The expansion and/or reaction step can occur in a closed or openenvironment. For example, expansion can take place in a closed mold. Theexpansion and/or reaction step may also occur in an open system, such ason a web upon which the stable dispersion has been applied, for example,by extrusion or printing. The dispersion may be applied to a temporaryor permanent substrate prior to its expansion and reaction to form thepolymer foam.

As previously stated, the substantially water-insoluble polymer isreacted to form a substantially water-insoluble, network crosslinkedpolymer. The reaction is typically caused by heating the expandedsubstantially water-insoluble polymer structure to the activationtemperature. As noted above, the activation temperature may be equal tothe critical temperature, in which case crosslinking coincides withinsoluble polymer formation. In such case, no additional reaction isgenerally required (although further crosslinking may be used to modifyabsorptive properties).

Conditions for reacting the substantially water-insoluble polymer toform a network crosslinked polymer may be selected consistent with theabove teachings from polymerization conditions such as are generallydescribed in the above referenced U.S. Pat. No. Re. 32,649 and in U.S.Pat. No. 4,666,983 issued to Tsubakimoto et al.; and U.S. Pat. No.4,62S,001 issued to Tsubakimoto et al.. Such reaction conditionsgenerally involve heating (i.e., thermal activation techniques) thepolymer to a temperature of from about 0° C. to about 100° C., morepreferably from about 20° C. to about 80° C., most preferably from about50° C. to about 80° C. Temperatures of less than about 80° C. arepreferred for obtaining foams having a low extractable polymer content.Conditions under which the polymer is maintained can also include, forexample, subjecting the polymer to any conventional form ofpolymerization activating radiation. Radioactive, electronic,ultraviolet, or electromagnetic radiation are suitable alternativeconventional polymerization techniques.

"Network" crosslinking refers to the reaction of reactive sites of themonomer or polymer thereof with the reactive sites of the internalcrosslinking agent to form a three-dimensional polymer network in whichthere are crosslink bonds between different polymer chains, the polymerchains being generally attributable to polymerization of the monomer.Sufficient network crosslinking provides a substantially water-insolublepolymer. Sufficient network crosslinking, which may or may not be thesame degree as required to provide substantial insolubility, alsorenders the polymer superabsorbent (where a sufficient number ofneutralized carboxyl groups are present in the polymer). Thus,additional crosslinking of the insoluble polymer material may berequired in order to render it superabsorbent. The level of crosslinkingof the insoluble polymer material is generally determined by thereactant type (particularly the crosslinking agent) and concentrationand the reaction conditions. Therefore, if necessary, the substantiallywater-insoluble polymer material can be made superabsorbent by exposingit to more extreme reaction conditions and/or additional crosslinkingagent. (Since superabsorbency is also influenced by the level of saltgroups in the polymer, various degrees of superabsorbency may also beimparted by neutralizing at least a portion of the salt-forming groups(e.g., carboxyl groups) of the polymer as previously described.)

The degree of network crosslinking, i.e., the crosslink density, may bevaried as known in the art to provide various gel strengths and gelvolumes as may be desired. In general, as the crosslink densityincreases, the gel strength increases, gel volume decreases and, at aconstant surface area to mass ratio, the rate of absorption decreases.Thus, the degree of network crosslinking in part serves to determine theabsorptive capacity and absorptive rate of the superabsorbent polymermaterial and thus of the superabsorbent polymer foam. The degree ofnetwork crosslinking also influences the residual monomer andextractable polymer content of the foam. In a preferred embodiment, theinternal crosslinking agent and monomer are reacted so as to form asubstantially water-insoluble superabsorbent polymer material which ispartially network crosslinked so as to achieve a desired gel strengthand gel volume.

In the preferred embodiment in which the solvent is water and theexpansion and reaction has occurred at temperatures less than theboiling point of water (i.e., 100° C. at 1 atm), the resultant foam willtypically comprise water as absorbed from the reaction mixture by thesuperabsorbent polymer material of the foam. Some level of water isgenerally desired in the foams in order to enhance the flexibility andabsorptive rate of the foam. S Preferably, the foam will contain lessthan about 20%, more preferably less than about 10%, most preferablyless than about 5% by weight of water per 100 parts by weight of thesuperabsorbent polymer material of the foam. (These preferredpercentages include any water which may be absorbed from theenvironment.) The level of water may be controlled by varying the amountof water used in the reaction mixture. In addition, water may be addedto the superabsorbent polymer foam (e.g., as an external plasticizer).Although it will usually be unnecessary, the water content may bedecreased by, for example, displacing the water with a lower alcoholsuch as those described herein and/or by heating the foam to evaporatethe water, e.g., by microwave radiation.

Where the solvent is other than water, any remaining solvent may beremoved in whole or part from the resultant foam. This is most readilycarried out by heating the foam material to a temperature sufficient toevaporate the solvent within a reasonable time, e.g., near or above theboiling point of the solvent.

Typically, the components of the reaction mixture and the blowing agentare introduced into a vessel equipped for high shear mixing, and thestable dispersion as described above is formed in the vessel. Theblowing agent is then expanded and the monomer and internal crosslinkingagent are reacted to form the foam. The foam is typically shaped duringand/or after its formation. Shaping may be achieved by any conventionalshaping techniques as are known in the art to form a foam having adefined shape and size. Preferred methods for shaping the foam includecasting, molding, or forming operations.

Casting and molding techniques generally involve introducing the stabledispersion into a prepared mold cavity (open or closed mold) and causingexpansion and reaction such that the foam conforms to the shape of themold cavity. Examples of specific molding techniques for use hereininclude compression molding, injection molding, extrusion, orlaminating. Forming techniques involve performing various operations onthe stable dispersion or foam to modify its shape and/or size. Examplesof specific forming techniques for use herein include grinding,chopping, cutting, coating and extruding operations. For example, thestable dispersion may be extruded through an orifice to form a foamhaving a shape corresponding to the shape of the orifice. Further, thestable dispersion may be cast on a surface to form a foam having adesired shape or surface morphology. Any or all of these techniques mayalso be used in combination to form the shaped foam. Any suitableapparatus as are known in the art may be used to carry out suchoperations.

The superabsorbent polymer foams of the present invention are useful infree form, including particulate (includes granules, chunks, and thelike), sheet or other three-dimensional forms. A particulate foammaterial can be obtained from a bulk sample of foam material by anysuitable method, e.g., chopping or grinding. However, in any suchprocess it will generally be desired to make efforts to substantiallypreserve the morphology (e.g., surface area to mass ratio) of the foammaterial as originally formed. To form a free (i.e., unsupported) foamsheet, the stable dispersion is applied to a temporary substratefollowed by expansion of the blowing agent and reaction of the internalcrosslinking agent with the monomer to obtain the substantiallywater-insoluble, polymer foam. The foam is then readily removed from thetemporary substrate. Temporary substrates include any materials knownfor such purpose, e.g., TEFLON® sheets, MYLAR® sheets, andrelease-coated metal sheets. The stable dispersion may be applied to thesubstrate by any conventional method of preparing films or prints, forexample, knife-coating, spray-coating, reverse-roll coating,gravure-coating, cold extrusion coating or casting, and the like. Thestable dispersion may be applied to the substrate to obtain a foamproduct in a desired shape. Alternatively, the foam product may be cutto a desired form.

The superabsorbent polymer foams of the present invention are alsouseful when joined to a carrier to form an absorbent member. Thecarriers may be any carriers as are known in the art such as nonwovenwebs, tissue webs, conventional foams, polyacrylate fibers, aperturedpolymeric webs, synthetic fibers, metallic foils, elastomers, and thelike. The carrier may be any material generally employed for use inabsorbent articles, including wicking and non-wicking materials.Examples of wicking carriers include tissue paper (which includes papertoweling). Examples of non-wicking carriers include polymer films suchas polypropylene. For use in absorbent articles, a particularly suitablecarrier is paper tissue. For example, suitable paper tissue is disclosedin U.S. Pat. No. 4,191,609 issued to Trokhan on Mar. 4, 1980; U.S. Pat.No. 4,529,480 issued to Trokhan on Jul. 16, 1985; and U.S. Pat. No.4,637,859 issued to Trokhan on Jan. 20, 1987, each incorporated hereinby reference. Suitable carriers also include any of the backsheetmaterials described herein for use in absorbent articles. The carriermay be of any desired shape and may be shaped before, during or afterjoinder with the foam.

The foam may be joined to the carrier via chemical or physical bondingmethods such as are known Including adhesives or chemicals that react toadhere the foam to the carriers. Joinder is also meant to includegenerally non-bonded combinations of the foam with other materials,e.g., encasing or sandwiching the foam in or between other materials.Alternatively, the foam can be joined to the carrier by applying thestable dispersion to the carrier and causing expansion and reaction soas to form the foam on the carrier (i.e., the carrier is a permanentsubstrate) to provide a superabsorbent foam-bearing structure. The foamsas joined to the carrier by any one of these methods can be in acontinuous or discontinuous form or pattern.

Where the foam is formed on the carrier (i.e., the carrier is apermanent substrate), the carrier and foam material are preferablyselected such that there is sufficient bonding (physical and/orchemical) of the foam to the carrier as required by the intendedapplication. The stable dispersion can be applied to the entire surfaceof the carrier or to any portion thereof and in any continuous ordiscontinuous pattern. Suitable patterns can be selected for tailoringthe absorption properties of the foam-bearing structure as may bedesired. For example, for use in disposable diapers, particular patternsmay be formed to provide defined absorption properties in diapers forfemale or male use.

In a preferred embodiment of the present invention, the stabledispersion is transferred to and extruded through a conventionalextruder apparatus. An example of an extruder apparatus is shown inFIGS. 12-14 of Principles of Polymer Materials, Second Edition(McGraw-Hill Book Co., 1982) at page 331, which is incorporated hereinby reference. The stable dispersion is extruded through the orifice ofthe extruder apparatus onto a temporary or permanent web substrate,followed by the expansion and .reaction steps to form a free foam sheetor a foam-bearing structure. The temperature and pressure can becontrolled in the extruder so as to control the expansion and reactionprocess. For example, polymerization of the monomer to form asubstantially linear polymer may be caused while the mixture is in theextruder. Expansion and reaction of the linear polymer, any remainingmonomer, and the internal crosslinking agent to form a substantiallywater-insoluble foam may then be caused when the dispersion is at orbeyond the orifice.

In another preferred embodiment, the stable dispersion is printed onto apermanent substrate followed by expansion and reaction to form afoam-bearing substrate. Exemplary printing apparatus and techniquesinclude rotogravure and flexographic printing equipment and processessuch as disclosed in Flexography Principles and Practices, 4th Edition,Flexographic Technical Association, Inc., 1991, incorporated herein byreference. The permanent substrate may be any of those discussed herein,preferably a wicking material, more preferably comprising cellulosicfibers, most preferably wood pulp fibers.

Where a wicking substrate is used, it is particularly desirable to printthe stable dispersion in a discontinuous pattern on the wickingsubstrate in order to form an absorbent member possessing both thecapillary absorptive properties of the wicking material and the osmoticabsorptive properties of the resultant discontinuous pattern ofsuperabsorbent foam. To form the discontinuous pattern, it is generallydesirable to use a stable dispersion having a sufficiently highviscosity such that wicking of the dispersion into the substrate doesnot occur to a substantial extent. In this way, the wicking propertiesof the substrate are substantially maintained in addition, the higherviscosity may prevent or minimize partitioning of the solvent and thereactants. The viscosity can be controlled by any of the methodsdiscussed herein in reference to forming the stable dispersion, e.g.,through the use of a viscosity control agent and/or multi-step reactionprocess. In addition, it is generally desirable to initiate expansionand reaction to form the foam within a short time, preferablyimmediately, after printing of the stable dispersion onto the substrate.For example, polymerization of the monomer to form a linear polymer maybe initiated by, e.g., subjecting the printed dispersion to ultravioletradiation. Expansion and reaction of the linear monomer, any remainingmonomer, and the internal crosslinking agent in the printed dispersionis then caused, e.g., by subjecting the printed dispersion to heat. Theultraviolet radiation used to initiate polymerization of the monomer maybe sufficient where the blowing agent has a sufficiently low boilingpoint and the crosslinking agent has a sufficiently low activationtemperature. If necessary, additional sources of heat may be used, e.g.,infrared radiation.

In an especially preferred embodiment, the superabsorbent polymer foamof the present invention is flexibilized. Flexibilization may beachieved by including suitable "internal" plasticizers in the reactionmixture, the plasticizers being reactive with at least one of thesuperabsorbent-polymer-forming materials. Certain internal plasticizersmay be used as a monomer component of the reaction mixture, eithersolely or in admixture with other monomers described above, providedthat they meet the description of those monomers. Such monomers can beselected by one having ordinary skill in the art in view of thisdisclosure.

The internal plasticizers include unsaturated materials capable ofreacting under the above-described polymerization conditions to form anaddition-type polymer material, or the addition polymer itself. Theaddition polymers will have a relatively low glass transitiontemperature (i.e., Tg), e.g., less than about 25° C. The plasticizer maypossess hydrophilic groups such as acid or other functional groups.Suitable internal plasticizers include olefins, aromatic ethylenicallyunsaturated monomers, and C1-C24 alkyl esters of unsaturated carboxylicacids. Preferably, the internal plasticizer is isobutylene or 2-ethylhexylacrylate.

The type and level of internal plasticizer can be selected by one havingordinary skill in the art in order to obtain a superabsorbent polymerfoam having various degrees of flexibility. Preferably, the foam isdesigned such that an absorbent article such as a diaper or femininehygiene product is sufficiently compliant so as to readily conform tothe general shape and contours of the wearer's body.

In place of or in addition to the internal plasticizer, an "external"plasticizing compound may be used to provide flexibility in theresultant superabsorbent polymer foam. External plasticizers generallyinclude plasticizing compounds other than the above addition polymersand addition-polymer-forming materials. Such plasticizers includehydrophilic compounds and hygroscopic compounds. Exemplary hydrophiliccompounds are water and relatively higher molecular weight polyols suchas polyethylene glycol and polypropylene glycol having a weight averagemolecular weight of about 600 grams/mole. Preferably, a hygroscopiccompound is used. Exemplary hygroscopic compounds are glycerol andrelatively low molecular weight polyols such as polyethylene glycol andpolypropylene glycol having a weight average molecular weight of about200 grams/mole. Host preferably, the external plasticizer is glycerol.

The external plasticizers may or may not be reactive or reacted with therequired superabsorbent-polymer-forming materials. The plasticizer maybe included in the above described reaction mixture. Alternatively, thesuperabsorbent polymer foam may be treated with the external plasticizerafter formation of the foam, e.g., by spraying or immersion.

The specific amounts of external plasticizer may be selected by onehaving ordinary skill in the art to flexibilize the superabsorbentpolymer foam to the extent desired. For example, where the plasticizeris included in the reaction mixture and is reactive with one or more ofthe superabsorbent-polymer-forming materials, the plasticizer may beused in a stoichiometrically excessive amount in order to obtain thedesired flexibility. Alternatively, the external plasticizer may beadded to the reaction mixture which is then reacted under conditionsinsufficient to cause full reaction of the plasticizer with the requiredsuperabsorbent-polymer-forming materials.

Where the substantially water-insoluble polymer material of the polymerfoam is not crosslinked or only partially crosslinked, the foam may befurther reacted so as to crosslink or further crosslink the polymermaterial. As noted above, some degree of crosslinking is necessary torender the polymer material superabsorbent. In addition, furthercrosslinking may also be desired in order to impart certain absorptiveproperties to a polymer foam which is already superabsorbent.

An uncrosslinked foam may result where the substantially water-insolublepolymer material comprises only the monomer in polymerized form, e.g.,where the reaction mixture does not include an internal crosslinkingagent or does include a latent crosslinking agent. Latent crosslinkingagents may also result in partial crosslinking. By "latent crosslinkingagent", it is meant that the crosslinking agent will not react under theparticular conditions used for reacting the monomer and the internalcrosslinking agent to form the water-insoluble polymer of the polymerfoam. Thus, the previously described internal crosslinking agents may beused as latent crosslinking agents as long as the reaction conditionsare selected so as to ensure latent reaction. However, the reactionmixture will preferably include at least one internal crosslinking agentwhich is reacted to form the substantially water-insoluble polymermaterial.

Where a latent crosslinking agent has been used, the foam may besubjected to conditions of time and temperature so as to react thelatent crosslinking agent into the polymer network of the substantiallywater-insoluble polymer material of the foam. Although the requiredreaction conditions will vary, for example, with the particularchemistry involved, the reaction generally involves heating of the foamat temperatures of from about 100° C. to about 250° C. for a time ofabout 1 minute to about 30 minutes. Such reaction typically causesnetwork crosslinking in substantially all of the polymer material of thefoam. As a result, it typically increases the gel strength (anddecreases the gel volume) of substantially all of the polymer materialof the foam.

Partial crosslinking may also result from the reaction of only a portionof the monomer reactive sites with the internal crossl inking agent,due, for example, to a stoichiometric deficiency of internal crosslinking agent in forming the substantially water-insoluble polymer foam.The polymer material of such a foam (which may already besuperabsorbent) can be further crosslinked after formation of the foamby reaction of the polymer material with a suitable externalcrosslinking agent. External crosslinking results in crossl inking ofthe polymer material of the foam surfaces in contact with thecrosslinking agent, and depending on the reaction conditions, thesubsurface polymer material. In general, the more extreme the conditionsor reactive the reactants, the more surface and subsurface polymermaterial will be crosslinked. Crosslinking can be caused through variousdepths of the polymer material of the polymer foam, from the polymersurfaces in contact with the external crosslinking agent into theinterior of the polymer material. In addition, zones of differentcrosslink densities can be prepared, e.g., by causing crosslinking in astepwise manner. Thus a crosslink gradient can be formed with thehighest crosslink density being toward the polymer surfaces (includingthose of the intercommunicating channels). In a preferred embodiment,only the surfaces of a foam formed of superabsorbent polymer materialare further crosslinked such that the foam surfaces, including thesurfaces of the intercommunicating channels, have relatively high gelstrengths while the gel strength and gel volume of the subsurfacepolymer material is generally unchanged. Without wishing to be bound bytheory, it is believed that such crosslinking may provide a gel strengthsufficient to provide surface dryness of the foam when wetted (andtherefore skin dryness in use) without significant adverse impact on theosmotic absorptive capacity and rate. In addition, such surfacecrosslinking may enhance the capillary absorptive rate.

Suitable external crosslinking agents include those described herein inreference to the internal crossl inking agent. Preferably, the externalcrosslinking agent is capable of forming covalent crosslink bonds withthe polymer material of the foam, i.e., the external crossl inking agentcontains one or more functional groups or unsaturated groups that arereactive with the polymer material of the foam. Where the substantiallywater-insoluble polymer material contains carboxylic acid groups, thepreferred embodiment of covalent crosslinking generally occurs as aresult of the formation of ester, amide, imide or urethane groups byreaction of the carboxylic acid groups with the corresponding functionalgroup of the crosslinking agent. Accordingly, preferred crosslinkingagents include polyhydroxy compounds, polyamines, polyisocyanates,polyamides, polyepoxides and hydroxyepoxide compounds. Preferably, theexternal crosslinking agent is a polyhydroxy compound, e.g.,polyethylene glycol, polypropylene glycol, and glycerol. Mostpreferably, glycerol is used.

The polymer foam can be reacted with the external crosslinking agent byfirst exposing the foam to the external crosslinking agent, by, forexample, spraying, immersion, or vapor deposition. The polymer materialof the foam is then reacted with the external crosslinking agent underconditions of time and temperature sufficient to cause the reaction tothe extent desired, depending on the particular polymer material andexternal crosslinking agent. Such conditions may be readily determinedby one skilled in the art. Depending on the reaction chemistry, thecrosslinking reaction may occur spontaneously upon contacting thepolymer foam with the crosslinking agent, but generally has to beinduced by, for example, irradiation or heating. Therefore, the externalcrosslinking agent will be chosen with consideration given to the methodof induction.

Some of the external crosslinking agents, such as polyhydroxy compounds,can also serve as external plasticizers. In general, any externalcrosslinking agent of this type which is not fully reacted into thesuperabsorbent polymer material will serve as an external plasticizer.This can be achieved by using a stoichiometric excess of externalcrosslinking agent to polymer material, or by controlling the externalcrosslinking reaction conditions so as to avoid complete reaction of thecrosslinking agent with the polymer material. In addition, the waterwhich may be attracted by a hygroscopic crosslinking agent, for example,glycerol, may act to plasticize the superabsorbent polymer foam.

The resultant superabsorbent polymer foams of the invention can becharacterized by various absorbency, structural, mechanical and otherproperties as follows:

ABSORBENCY CHARACTERISTICS

Absorptive capacity refers to the capacity of the superabsorbent polymerfoam to absorb and retain fluids with which it comes into contact.Absorptive capacity of the foams herein can be considered to have twocomponents: an osmotic absorptive capacity (i.e., gel volume) and acapillary absorptive capacity. The osmotic absorptive capacity refers tothe ability of the polymer material of the foam to absorb fluids,whereas the capillary absorptive capacity refers to the ability of thefoam channels to absorb fluids (e.g., by wicking). Similarly, the foamsherein have an osmotic absorptive rate (the rate at which the polymermaterial absorbs fluids) and a capillary absorptive rate (the rate atwhich the channels absorb fluids). Unless otherwise specified, as usedherein the absorptive capacity and rate refer to the total (i.e.,combined osmotic and capillary) absorptive capacity and rate.

Absorptive capacity can vary significantly with the nature of the fluidbeing absorbed and with the manner in which fluid contacts the foam. Forpurposes of this invention, absorptive capacity is defined in terms ofthe amount of synthetic urine absorbed by any given superabsorbentpolymer foam material in terms of grams of synthetic urine per gram offoam in a procedure hereinafter defined (since the specific gravity ofthe synthetic urine is approximately 1.0, absorptive capacity can alsobe reported in terms of ml of synthetic urine per gram of foam).

The superabsorbent polymer foams of the present invention are thosewhich have an absorptive capacity of at least about 10 grams, morepreferably at least about 15 grams, most preferably at least about 50grams, of synthetic urine per gram of foam. Superabsorbent polymer foamshaving this relatively high absorptive capacity characteristic areespecially useful in absorbent structures and articles since the foamscan, by definition, hold desirably high amounts of discharged bodyfluids such as urine.

The absorptive rate is the absorptive capacity measured as a function oftime. The absorptive rate, in grams (g) of synthetic urine per gram ofsuperabsorbent polymer foam per second (sec), is preferably at leastabout 0.5 g/g/sec, more preferably at least about 1 g/g/sec, mostpreferably at least about 2 g/g/sec.

Absorptive capacity and rate can be determined for any given foam sampleusing the procedure described in the TEST METHODS section.

In addition to a relatively high absorptive capacity and absorptiverate, the superabsorbent polymer foams preferably also possess certaingel strength characteristics. Gel strength refers to the propensity ofthe foam to deform or spread under stress once the foam absorbs fluid.Without wishing to be bound by theory, it is believed that, for a givensuperabsorbent polymer material and gel strength, the capillaryabsorptive capacity and the capillary and osmotic absorptive rates maybe increased by increasing the surface area to mass ratio of the foam.Therefore, it is desirable to utilize in absorbent structures andarticles those superabsorbent polymer foams having an adequate gelstrength and the greatest achievable surface area to mass ratio.

It has also been found that gel strength (i.e., gel deformationtendency) correlates directly with the shear modulus of thesuperabsorbent polymer foam. Accordingly, superabsorbent polymer foamshaving sufficient gel strength to be useful in absorbent structures andarticles of the present invention can be appropriately characterized byspecifying gel strength in terms of the shear modulus of the foam.

Shear modulus can be conventionally measured, for example, by aprocedure which involves the use of a stress rheometer to determine theratio of (a) stress applied to a given foam sample to (b) the resultantstrain exhibited by the sample. The foam sample tested in this manner isswollen to its absorptive capacity with synthetic urine. The stress tostrain ratio is determined, and the shear modulus of the resultant foamsample in dynes/cm² is then subsequently calculated from this ratio. Asuitable procedure for use herein is described in U.S. Pat. No. Re.32,649 issued to Brandt et al. on Apr. 19, 1988, incorporated herein byreference.

The superabsorbent polymer foams of the present invention preferablyhave gel strengths such that these foams exhibit a shear modulus of atleast about 1 dyne/cm², more preferably within the range of from about 1dyne/cm² to 5 dynes/cm². Without being bound by any particular theory,it is believed that foams having such gel strengths will resistdeformation upon fluid absorption and will have a reduced tendency toflow (i.e., the foams are "fluid stable"). Thus, the preferred gelstrengths may allow the intercommunicating channels of the foams of thepresent invention to be maintained and enlarged when swollen by fluidsso that the foam may acquire and transport subsequent loadings offluids. The preferred gel strengths may also serve to enhance skindryness.

Another feature of the foams of the present invention is that the foamsswell generally isotropically, even under moderate confining pressures,when fluids are deposited onto or come into contact with the foams.Isotropic swelling is used herein to mean that the foam swells generallyequally in all directions when wetted. Isotropic swelling is animportant property of the foam because the superabsorbent polymermaterial of the foam, cells, and intercommunicating channels are able tomaintain their relative geometry and spatial relationships even whenswollen such that the existing capillary channels are maintained, if notenlarged, during use (the polymer material, cells and channels getlarger during swelling). Thus, the foam can imbibe and/or transportthrough itself additional loadings of fluid.

STRUCTURAL FEATURES

Specific, somewhat interrelated and interdependent structural propertiesof the superabsorbent polymer foams have been identified as being highlydesirable in applications involving absorption of aqueous body fluids.The several structural properties of the preferred superabsorbentpolymer foams can be summarized as follows:

A) SURFACE AREA TO UNIT MASS RATIO

The surface area to unit mass ratio is the total area of thesuperabsorbent polymer material surfaces of the foam, including thesurfaces of the cells and intercommunicating channels, to the total massof the superabsorbent polymer material. The surface area to mass ratiois indicative of the rate of fluid uptake, particularly the osmoticabsorptive rate, of the foam. The greater the surface area to mass ratioof the foam, the more area there is for diffusion of the fluid to beabsorbed. Thus, for foams having a given gel strength characteristic,foams having a higher surface area to mass are preferred.

The surface area to mass ratio is also believed to be important inminimizing deactivation of the superabsorbent material of the Foam bybody fluids containing high molecular weight components, e.g., blood andmenses. If the surface area to mass ratio is too small, such components,due to their molecular size, are believed to physically deactivate thefoam or portions thereof.

For the above reasons, it is desirable to maximize the surface area tomass ratio of the superabsorbent polymer foams of the present invention.In general, any feature which increases foam capillarity will alsoincrease the surface area to mass ratio. For example, surface area tomass increases as the cell size decreases and/or as the number of cells,percent open cells, and/or percent intercommunicating channelsincreases. Thus, the surface area to mass ratio may be increased by anyof the foam composition or processing parameters which so influencethese parameters.

The superabsorbent polymer foams herein will typically have a surfacearea to mass ratio of at least about 0.2 m² /gram, more preferably atleast about 1.6 m² /g, and most preferably at least about 3 m² /gram. Asuitable method for determining the surface area to mass ratio (usingthe Brunauer-Emmet-Teller (BET) gas adsorption method) is set forth ingreater detail in the TEST METHODS section.

B) PERCENT OPEN CELLS

Polymeric foams may be relatively closed-celled or relativelyopen-celled in character, depending upon whether and/or the extent towhich, the cell walls or boundaries (i.e., the cell windows) are filledor taken up with polymeric material. The superabsorbent polymer foams ofthe present invention are relatively open-celled in that the individualcells of the foam are for the most part not completely isolated fromeach other by polymeric material of the cell walls. Thus, the cells insuch substantially open-celled foam structures have intercellularopenings or "windows" which are large enough to permit ready fluidtransfer from one cell to the other within the foam structure. Anopen-celled structure is important for both the capillary absorptiverate and capacity of the foam. Improved capillary transport al soimproves the osmotic absorptive rate since a greater area of thesuperabsorbent polymer material of the foam is exposed to the fluid.

In substantially open-celled structures of the type useful herein, thefoam will generally have a reticulated character with S the individualcells being defined by a plurality of mutually connected, threedimensionally branched webs. The strands of polymeric material whichmake up the branched webs of the open-cell foam structure can bereferred to as "struts." For purposes of the present invention, asuperabsorbent foam is "open-celled" if at least about 25%, preferablyat least about 50%, and most preferably at least about 75% of the cellsin the foam structure are in fluid communication with at least oneadjacent cell. Alternatively, the foam may be considered to besubstantially open-celled if it has an available pore volume whichexceeds a minimum value as set forth hereinafter.

In addition, the intercellular openings of the foams herein are suchthat the foam has substantially continuous, intercommunicating channelssubstantially throughout the foam network, i.e., the intercellularopenings form an interconnecting network of channels which are free ofpolymer material such that the foam is liquid permeable. The channelsallow fluids contacting the foam to be transported via capillary forces(i.e., capillary transport channels are formed) to other portions of thefoam so that the total volume of the foam is used in absorbing thefluids. Further, when swollen, the cells and the intercommunicatingchannels allow fluids to pass through the foam either to superabsorbentpolymer material remote from the initial point of fluid contact or toother structures in contact with the foam. Thus, the foam is consideredto be fluid permeable due to the cells and the intercommunicatingchannels. The intercommunicating channels are believed to enhance thecapillary absorptive rate and capacity. Since the channels allowdistribution of fluids throughout the foam, they also provide increasedutilization of the fluid-retentive properties of the superabsorbentpolymer of the foam, thereby enhancing the osmotic absorptive rate ofthe foam.

Preferably, the intercellular openings are such to allow intercellularcapillary flow of fluids (i.e., intercellular fluid communication) amongat least about 25%, more preferably at least about 50% percent, mostpreferably at least about 75% percent, of the cells. The surface area tomass ratio and the available pore volume may be indicative of the levelof cells having intercellular communication.

Features which influence the formation of intercommunicating channelsinclude the level, particle size, and particle size distribution of theblowing agent and control of the expansion and reaction steps so as toexpand the blowing agent prior to the critical viscosity.

C) AVERAGE CELL SIZE AND CELL SIZE DISTRIBUTION

Another structural feature of the superabsorbent polymer foams herein iscell size. Foam cells will frequently be substantially spherical inshape. Thus, the size or "diameter" of such substantially sphericalcells is a commonly utilized parameter for characterizing foams ingeneral, as well as for characterizing the preferred superabsorbentpolymer foams of the present invention. Since cells in a given sample ofpolymeric foam will not necessarily be of approximately the same size,an average cell size, i.e., average cell diameter, will often bespecified. Similarly, a cell size distribution reflecting the range ofcell sizes may be specified.

Since cell size is a factor that determines the capillarity of the foam,cell size is a parameter that can directly affect both the osmotic andcapillary absorptive rates of the superabsorbent foams herein. Cellsize, in conjunction with the number of cells (which relates to density)may also affect mechanical properties, including flexibility, of thefoams herein. In general, for a given density, the overall flexibilityof the bulk foam may increase as the average cell size decreases.

A number of techniques are available for determining average cell sizein foams. These techniques include mercury porosimetry methods which arewell known in the art. The most useful technique, however, fordetermining cell size in foams involves simple photographic measurementof a foam sample. FIG. 1 of the drawings, for example, is aphotomicrograph of an edge of a typical superabsorbent foam of thepresent invention. Superimposed on the portion of the photomicrographmarked 1a (the edge enlarged about 50 times) is a scale representing adimension of 50 mm; on the portion marked 1b (a rectangular portion of1a enlarged about 250 times) a scale representing a dimension of 100microns (Similarly, FIG. 2, a photomicrograph of an edge of the foam ofFIG. 1 taken from a plane perpendicular to the plane of the edge in FIG.1, has similar scales superimposed on the portions marked 2a (50X) and2b (250X)). Such scales can be used to determine average cell size viaan image analysis procedure. Image analysis of photomicrographs of foamsamples is a commonly employed analytical tool which can be used todetermine average cell size of the foam structures herein. Such atechnique is described in greater detail in U.S. Pat. No. 4,788,225issued to Edwards et al. on Nov. 29, 1988, incorporated herein byreference. From the various cell measurements which are taken indetermining average cell size, a cell size distribution can be easilydetermined, including range and size percentages within such range.

It is generally desirable to provide foams having the minimum possiblecell size consistent with obtaining the intercellular fluidcommunication and surface area to mass ratio previously described. Asdetermined by direct photographic measurement, the superabsorbentpolymer foams of the present invention will typically have an averagecell size of less than about 100 microns, more preferably less thanabout 50 microns, most preferably less than about 20 microns. Generallythe minimum cell size will be in the range of from about 1 to about 10microns. Preferably, at least about 50%, more preferably at least about75%, most preferably at least about 90%, of the cells will lie withinthe above cell size ranges.

The cell size distribution may affect both the osmotic and capillaryabsorptive rates. For maximizing the capillary absorptive rate it isbelieved preferable to provide a narrow cell size distribution,preferably such that the foam has a cell distribution value of at most5, more preferably at most 3, still more preferably at most 2. For thefastest capillary absorptive rate, the cell distribution value ispreferably 1 (i.e., all cells are of the same size). Cell sizedistributions corresponding to these values are summarized in Table I.It is further believed that the osmotic absorptive rate is maximizedwith a foam morphology having combination of small cells and a widercell size distribution such that there is "packing" of smaller cellsamong larger cells. Therefore, for a suitable balance of both osmoticand capillary absorption by the foams herein it is preferred that thecell distribution value be greater than 1 and less than 3, mostpreferably less than 2.

The size of the cells in the superabsorbent polymer foams can beinfluenced and controlled by variation of certain foam composition andprocessing features as described herein, particularly those whichinfluence the particle size of the blowing agent in the stabledispersion. For example, the cell size is influenced by the viscosityratio, the shear stress in forming the dispersion, and the temperatureand pressure used in the expansion and reaction steps. For sufficientlyflexible foams, cell size may also be altered by simply compressing thesolid foam structures after they have been prepared.

A narrow cell size distribution may be promoted by proper selection of asurfactant for a given blowing agent and reaction mixture so as tominimize coalescence of the dispersed blowing agent particles beforeand/or during expansion. For example, a surfactant containinghydrophilic and hydrophobic groups is generally effective forstabilizing a water-insoluble blowing agent in a reaction mixture inwhich the solvent is water. The ratio of hydrophilic and hydrophobicgroups in the surfactant can be varied to minimize coalescence of theblowing agent particles, preferably by matching the HLB values of thesurfactant and the blowing agent, thereby promoting a uniform cell sizedistribution. A narrow cell size distribution may also be promoted bydecreasing the viscosity ratio.

                                      TABLE I                                     __________________________________________________________________________    Average                                                                       Cell  Cell Distribution Value*                                                Size  1     2        3        4        >5                                     __________________________________________________________________________    100 microns                                                                         100% - 100                                                                          50% - 95-105                                                                           33.3% - 95-105                                                                         25% - 95-105                                                                           17% - 95-105                                       25% - 75-<95                                                                           33.3% - 65-<95                                                                         37.5% - 62-<95                                                                         41.5% - <95                                        25% - >105-125                                                                         33.3% - >105-135                                                                       37.5% - >105-138                                                                       41.5% - >105                            50 microns                                                                         100% - 50                                                                           50% - 45-55                                                                            33.3% - 45-55                                                                          25% - 45-55                                                                            17% - 45-55                                        25% - 35-<45                                                                           33.3% - 35-<45                                                                         37.5% - 33-<45                                                                         41.5% - <45                                        25% - >55-65                                                                           33.3% - >55-65                                                                         37.5% - >55-65                                                                         41.5% - >55                             20 microns                                                                         100% - 20                                                                           50% - 18-22                                                                            33.3% - 18-22                                                                          25% - 18-22                                                                            17% - 18-22                                        25% - 15-<18                                                                           33.3% - 15-<18                                                                         37.5% - 12-<18                                                                         41.5% - <18                                        25% - >22-25                                                                           33.3% - >22-25                                                                         37.5% - >22-26                                                                         41.5% - >22                            __________________________________________________________________________     *Percent of cells having specified size, in microns (e.g., 25%  >105-125      means that 25% of the cells have a size of greater than 105 microns and       loss than or equal to 125 microns).                                      

D) FOAM DENSITY

Density of the superabsorbent polymer foams may influence a number ofperformance and mechanical characteristics of these foams. Suchcharacteristics include both the osmotic and capillary absorptivecapacities and rates, and foam flexibility. In general, for a givensurface area to mass ratio and cell size, as the foam density decreases,the osmotic absorptive rate and capillary absorptive capacity willincrease. Importantly, the density of the superabsorbent polymer foamscan also determine the cost effectiveness of the absorbent articlesherein.

Foam density (in grams of foam material per cubic centimeter of foamvolume in air) is specified herein on a dry basis. Thus the amount ofany absorbed aqueous liquid, e.g., any residual liquid which may be leftin the foam, for example, after its formation, is disregarded incalculating and expressing foam density. Foam density as specifiedherein does include, however, any residual solid material such assurfactant, external plasticizer, etc., which may be present in thesuperabsorbent polymer foam. Such residual material may, in fact,contribute significant mass to the foam material.

Any suitable gravimetric procedure which will provide a determination ofmass of solid foam material per unit volume of foam structure can beused to measure foam density. For example, a gravimetric proceduresuitable for use herein is described more fully in U.S. Pat. No.5,147,345 issued to Young et al. on Sep. 15, 1992. This patent isincorporated herein by reference. For those situations where the foamsample preparation procedures (drying, aging, preflexing, etc.,) mightinadvertently alter the density measurements obtained, then alternatedensity determination tests may also be utilized. Such alternativemethods, for example, might include gravimetric density measurementsusing a test liquid absorbed within the foam material. This type ofdensity determination method can be useful for characterizing very lowdensity foams such as the foams herein wherein the dry densityapproximates the inverse of the pore volume of the foam. See Chatterjee,"Absorbency," Textile Science and Technology, Vol. 7, 1985, p. 41,incorporated herein by reference. The ranges for foam density set forthhereinafter are intended to be inclusive, i.e., they are intended toencompass density values that may be determined by any reasonableexperimental test method.

It is generally preferred to minimize the density of the superabsorbentpolymer foams consistent with obtaining a foam which has a desiredstructure of intercommunicating channels, cell size, and surface area tomass ratio. Foam density can be adjusted by controlling certain foamcomposition and processing parameters, for example, the addition levelof the blowing agent. The superabsorbent polymer foams of the presentinvention will typically have dry basis density values which range fromabout 0.1 to about 0.5 g/cm³. Density of the superabsorbent polymerfoams herein need not be uniform throughout the structure; i.e., someportions or zones of the foam may have relatively higher or lowerdensities than other portions or zones thereof. However, where the foamhas substantially continuous intercommunicating channels substantiallythroughout and a relatively small cell distribution value, the densitywill be substantially uniform throughout.

E) PORE VOLUME

As stated above, for very low density foams, dry density approximatesthe inverse of the pore volume. Therefore, pore volume may serve toprovide density measurements for the foams herein. In addition, porevolume can be correlated to the number of cells, the percentage of opencells, and the percentage of intercommunicating channels in the foamstructure. In general, as the pore volume increases, the number ofcells, number of open cells, and the degree of fluid intercommunicationof foam channels also increases.

Pore volume is a measure of the volume of the openings or cells in aporous foam structure per unit mass of solid material (i.e., polymerstructure plus any residual solids) which forms the foam structure. Porevolume can be important in influencing a number of performance andmechanical features of the superabsorbent foams such as described inreference to the foam density.

Pore volume can be determined by any suitable experimental method whichwill give an accurate indication of the actual pore volume of thestructure. Such experimental methods will generally involve themeasurement of the volume and/or mass of a test liquid which can beintroduced into the foam structure and which therefore is representativeof the volume occupied by the open cells of the foam. For this reasonthe pore volume parameter of the foams may also be referred to as"available pore volume."

One conventional way for determining available pore volumeexperimentally involves the introduction of a low surface tension liquidsuch as isopropanol into the foam structure from outside the foamstructure. A procedure for determining available pore volume usingisopropanol is set forth in the above referenced U.S. Pat. No.5,147,345. It should be understood, however, that alternative testliquids and procedures may also be used to determine available porevolume.

It is generally desirable to maximize the pore volume of the foamsherein. The pore volume of the superabsorbent polymer foams can beinfluenced and controlled by adjusting many of the same foam compositionand processing parameters as for density adjustment. For example, porevolume influencing features may include the blowing agent addition leveland blowing agent particle size.

The superabsorbent polymer foams of the present invention will generallyhave a pore volume of at least about 5 ml/g, more preferably at leastabout 10 ml/g, and most preferably at least about 30 ml/g. Typically thepore volume will range from about 5 to about 40 ml/g. Such ranges forpore volume are intended to be an "inclusive" definition of theoreticalpore volume for the foams encompassed by this invention. Thus if anyexperimental method which can reasonably be expected to givemeasurements approximating theoretical pore volume provides valueswithin the foregoing ranges, then the foam materials tested by any suchmethod are within the scope of this invention.

F) CAPILLARY SUCTION SPECIFIC SURFACE AREA

Another structural feature of the preferred superabsorbent polymer foamsherein is a certain capillary suction specific surface area. Capillarysuction specific surface area is, in general, a measure of thetest-liquid-accessible surface area of the polymeric network forming aparticular foam per unit mass of the bulk foam material (i.e., polymerstructural material plus solid residual material). Capillary suctionspecific surface area is determined both by the dimensions (i.e.,diameter) of the cellular units in the foam and by the size (i.e.,length, width and thickness) of the struts which form cellular units.Capillary suction specific surface area is thus a way of quantifying thetotal amount of sol id surface provided by the foam network to theextent that such a surface participates in absorbency.

The capillary suction specific surface area of an open-celled foamstructure such as the superabsorbent polymer foams of the presentinvention is a feature of the foam that influences the capillarity (orcapillary suction) exhibited by the foam. Although the foam surface areato mass ratio is particularly important for determining the osmoticabsorptive rate of the foam, the foam capillarity may be selected andcontrolled such that the superabsorbent polymer foams have a degree ofcapillarity allowing transport, e.g., by wicking, of fluids within thefoam structure. Adjustment of capillary suction specific surface area isthus a suitable means for providing a degree of capillarity for thesuperabsorbent polymer foams as may be desired. Foams of relatively highcapillary suction specific surface area may provide a very desirablecombination of high capillary absorptive capacity and high capillary andosmotic absorptive rates. High specific surface area is a consequence ofthe fineness of the struts making up the foam structure.

The capillary suction specific surface area of the superabsorbent foamsis influenced and controlled by adjusting many of the same compositionand processing parameters which affect the foam density and pore volume.

For purposes of this invention, the specific surface area of any givensuperabsorbent polymer foam material can and will usually be determinedby a procedure which involves the principle of capillary suction. Insuch a procedure, capillary suction specific surface area is determinedby measuring the amount of capillary uptake of a low surface tensionliquid (e.g., ethanol) which occurs within a foam sample of a known massand dimensions. A detailed description of such a procedure fordetermining foam specific surface area via the capillary suction methodwhich is suitable for use herein is set forth in the above referencedU.S. Pat. No. 5,147,345. Any reasonable alternative method fordetermining capillary suction specific surface area may also beutilized.

The superabsorbent polymer foams herein preferably have a capillarysuction specific surface area of at least about 0.1 m² /g, morepreferably at least about 0.5 m² /g, most preferably at least about 1 m²/g. Typically the capillary suction specific surface area will be in therange of about 0.1 to about 5 m² /g. Superabsorbent foams having theabove capillary suction specific surface area values may possess anespecially desirable balance of capillary and osmotic absorptivecapacities and rates for aqueous body fluids such as urine.

MECHANICAL FEATURES

The superabsorbent polymer foams preferably possess mechanicalproperties, e.g., flexibility, integrity, softness, etc., which rendersuch foams especially suitable for use in absorbent articles such asdisposable diapers.

The superabsorbent foams of the present invention are preferablyflexible when intended for use in absorbent articles. By "flexible" itis meant that the foam material can be used in or as an absorbent memberwhich will conform to the general shape and contours of the human body.In general, flexible foams can be deformed or bent to the extentnecessary for use in such absorbent articles without significant damageto their structural integrity or significant loss of their absorbentproperties.

The superabsorbent polymer foams herein are also preferably sufficientlyflexible to withstand compressive or deforming forces which areencountered during preparation, processing, packaging, shipping andstoring of absorbent articles containing such foam materials. Disposablediapers, for example, are generally packaged and marketed in a foldedcondition wherein the diaper core is folded in both the longitudinal andtransverse directions. Disposable diapers are also generally marketed inthe form of stacks of folded diapers, which stacks are contained andcompressed by their surrounding packaging. Accordingly, the compressiveand deforming forces to which the superabsorbent polymer foam may besubjected during processing and marketing may be even greater than thosewhich are applied to the foam materials in use.

Given the nature of treatment which the superabsorbent polymer foamsherein must generally withstand, preferred foam materials of thisinvention will possess flexibility characteristics which can bequantified by referencing their ability to withstand bending withoutundergoing significant damage to their structural integrity. Describedin the TEST METHODS section is a procedure for determining theflexibility of the foams herein by determining whether and how manytimes a foam sample of a given specified size can be bent around acylindrical mandrel at a specified rate without breaking. Particularlypreferred foams of the invention are those which are flexible enough sothat, at their point of use as an absorbent for body fluids, the(synthetic urine) saturated foam material at 37° C. can be subjected tothis bending test without breaking (i.e., the foams exhibit a bendingvalue of at least one cycle). More preferably, the foams can be bent atleast 2 times, most preferably at least 5 times, without breaking whensubjected to such a test procedure.

The superabsorbent polymer foams of the present invention preferablypossess the additional mechanical attributes of structural integrity inuse and softness (i.e., lack of irritation) to the touch. For example,superabsorbent polymer foams that will be employed in such absorbentarticles as infant diapers will frequently be subjected to both dynamicand static forces when the wearer walks, runs, crawls or jumps. Suchforces may tend to rip or tear or otherwise fragment the foam structure.Thus, it would be advantageous for foams (particularly foam sheets)which are to be used in this manner to have sufficient structuralintegrity to minimize the incidence of foam tearing or fragmenting inuse.

The superabsorbent foams may also be used in absorbent articles inconfigurations wherein the foam material surface may come into closeproximity to the wearer's skin. Accordingly, it would be very desirablefor the surface of the superabsorbent foams herein to be acceptably softand non-irritating to the touch.

MISCELLANEOUS PROPERTIES

For use as fluid absorbents in absorbent articles, the superabsorbentpolymer foams herein preferably possess relatively low levels ofextractable polymer material. It is believed that polymer materialextracted by body fluid once the superabsorbent polymer material of thefoam forms a hydrogel can alter both the chemical and physicalcharacteristics of the body fluid to the extent that such fluid is moreslowly absorbed and more poorly held by the superabsorbent polymer foamcontaining absorbent article. Such effects are described in the abovereferenced U.S. Pat. No. Re. 32,649. As a result, leaching of polymerinto the body fluid to be absorbed can result in less efficientutilization of the superabsorbent polymer foam in the absorbent articlesand a greater incidence of undesirable leakage of body fluid from thearticle.

Extractable polymer levels of the superabsorbent polymer foams hereincan be determined by contacting a sample of the foam with a syntheticurine solution for the substantial period of time (e.g., at least 16hours) which is needed to reach extraction equilibrium, by thenfiltering the swollen foam material from the supernatant liquid, andfinally by then determining the polymer content of the filtrate. Such amethod for determining the extractable polymer content for carboxylicand sulfonic acid based superabsorbent polymer materials which issuitable for use herein is described in the above referenced U.S. Pat.No. Re. 32,649.

Preferred superabsorbent polymer foams of the present invention possesstwo types of extractable polymer content characteristics. It is believedthat both the total amount of extractable polymer in the foam and therate at which such extractable material is leached are important withregard to the absorptive performance of the superabsorbent polymer foam.Accordingly, the superabsorbent polymer foams herein preferably have aninitial extractable polymer content, i.e., the level of extractablepolymer which is removed after one hour in contact with synthetic urine,of no more that about 7.5% by weight, more preferably no more than about5% by weight of the foam. In addition, the foams preferably also have anequilibrium extractable polymer content, i.e., the equilibrium level ofextractable polymer removed after, for example, sixteen hours in contactwith synthetic urine, of no more than about 17% by weight, morepreferably no more than about 10% by weight of the foam sample.

In addition, preferred superabsorbent polymer foams of the presentinvention exhibit a particular relationship between absorptive capacityand equilibrium extractable polymer content, as described in the abovereferenced U.S. Pat. No. Re. 32,649. Such preferred foams can beprepared in accordance with the general teachings of U.S. Pat. Re.32,649 which are pertinent to such polymer materials. For example, theextractable polymer content may be minimized by a controlled, minimumutilization of initiator (when used) and/or the use of relatively lowtemperatures for reaction of the monomer and/or internal crosslinkingagent (e.g., from about 20° C. to about 80° C.).

APPLICATIONS

The superabsorbent polymer foams can be used for many purposes in manyfields of use as are known in the foam art. The foams are particularlysuitable for use in applications where it is desirable to absorb and/orretain fluids, for example, in absorbent articles; paper tissue(including toweling); packing containers; drug delivery devices; woundcleaning devices; burn treatment devices; ion exchange column materials;construction materials; agricultural or horticultural materials such asseed sheets or water-retentive materials; and industrial uses such assludge or oil dewatering agents, materials for the prevention of dewformation, dessicants, and humidity control materials.

Because of the unique absorption properties of the foam, thesuperabsorbent polymer foam is especially suitable for use as anabsorbent core material in absorbent articles, especially disposableabsorbent articles. As used herein, the term "absorbent article" refersto devices which absorb and contain body exudates, and, morespecifically, refers to devices which are placed against or in proximityto the body of the wearer to absorb and contain the various exudatesdischarged from the body. The term "disposable" is used herein todescribe absorbent articles which are not intended to be laundered orotherwise restored or reused as an absorbent article (i.e., they areintended to be discarded after a single use and, preferably, to berecycled, composted or otherwise disposed of in an environmentallycompatible manner). An "unitary" absorbent article refers to absorbentarticles which are formed of separate parts united together to form acoordinated entity so that they do not require separate manipulativeparts like a separate holder and liner. A preferred embodiment of anabsorbent article of the present invention is the unitary disposableabsorbent article, diaper 20 shown in FIG. 3. As used herein, the term "diaper" refers to an absorbent article generally worn by infants andincontinent persons that is worn about the lower torso of the wearer. Itshould be understood, however, that the present invention is alsoapplicable to other absorbent articles such as incontinent briefs,incontinent undergarments, diaper holders and liners, feminine hygienegarments, sanitary napkins, pantiliners, and the like.

FIG. 3 is a plan view of the diaper 20 of the present invention in itsflat-out, uncontracted state (i.e., with elastic induced contractionremoved) with portions of the structure being cut-away to more clearlyshow the construction of the diaper 20 and with the portion of thediaper 20 which faces away from the wearer, the outer surface 52,oriented towards the viewer. As shown in FIG. 3, the diaper 20preferably comprises a liquid pervious topsheet 24; a liquid imperviousbacksheet 26 joined with the topsheet 24; an absorbent core 28 (having agarment surface 100, a body surface 102, side edges 82, and waist edges83) positioned between the topsheet 24 and the backsheet 26; elasticizedside panels 30; elasticized leg cuffs 32; elastic waist features 34; anda fastening system 36.

The diaper 20 is shown in FIG. 3 to have an outer surface 52, an innersurface 54 opposed to the outer surface 52, a first waist region 56, asecond waist region 58 opposed to the first waist region 56, and aperiphery 60 which is defined by the outer edges of the diaper 20 inwhich the longitudinal edges are designated 62 and the end edges aredesignated 64. (While the skilled artisan will recognize that a diaperis usually described in terms of having a pair of waist regions and acrotch region between the waist regions, in this application, forsimplicity of terminology, the diaper 20 is described as having onlywaist regions, each of the waist regions including a portion of thediaper which would typically be designated as part of the crotchregion). The inner surface 54 of the diaper 20 comprises that portion ofthe diaper 20 which is positioned adjacent to the wearer's body duringuse (i.e., the inner surface 54 generally is formed by at least aportion of the topsheet 24 and other components joined to the topsheet24). The outer surface 52 comprises that portion of the diaper 20 whichis positioned away from the wearer's body (i.e., the outer surface 52generally is formed by at least a portion of the backsheet 26 and othercomponents joined to the backsheet 26).

FIG. 3 shows a preferred embodiment of the diaper 20 in which thetopsheet 24 and the backsheet 26 have length and width dimensionsgenerally larger than those of the absorbent core 28. The topsheet 24and the backsheet 26 extend beyond the edges of the absorbent core 28 tothereby form the periphery 60 of the diaper 20. While the topsheet 24,the backsheet 26, and the absorbent core 28 may be assembled in avariety of well known configurations, preferred diaper configurationsare described generally in U.S. Pat. No. 3,860,003 entitled"Contractable Side Portions for Disposable Diaper" which issued toKenneth B. Buell on Jan. 14, 1975; and U.S. Pat. No. 5,151,092,"Absorbent Article With Dynamic Elastic Waist Feature Having APredisposed Resilient Flexural Hinge", which issued to Kenneth B. Buell,et al. on Sep. 29, 1992; each of which is incorporated herein byreference. Alternatively preferred configurations for disposable diapersherein are also disclosed in U.S. Pat. No. 4,808,178 entitled"Disposable Absorbent Article Having Elasticized Flaps Provided WithLeakage Resistant Portions" issued to Mohammed I. Aziz and Ted L. Blaneyon Feb. 28, 1989; U.S. Pat. No. 4,695,278 entitled "Absorbent ArticleHaving Dual Cuffs" issued to Michael I. Lawson on Sep. 22, 1987; andU.S. Pat. No. 4,816,025 entitled "Absorbent Article Having a ContainmentPocket" issued to John H. Foreman on Mar. 28, 1989. These patents areincorporated herein by reference.

The topsheet 24 is positioned adjacent the body surface 102 of theabsorbent core 28. The topsheet 24 is compliant, soft feeling, andnon-irritating to the wearer's skin. Further, the topsheet 24 is liquidpervious, permitting liquids (e.g., urine) to readily penetrate throughits thickness. A suitable topsheet may be manufactured from a wide rangeof materials, such as porous foams; reticulated foams; apertured plasticfilms; or woven or nonwoven webs of natural fibers (e.g., wood or cottonfibers), synthetic fibers (e.g., polyester or polypropylene fibers), ora combination of natural and synthetic fibers. There are a number ofmanufacturing techniques which may be used to manufacture the topsheet24. For example, the topsheet 24 may be a nonwoven web of fibers, theweb being spunbonded, carded, wet-laid, meltblown, hydroentangled,combinations of the above, or the like, A preferred topsheet is cardedand thermally bonded by means well known to those skilled in the fabricsart, A preferred topsheet comprises a web of staple length polypropylenefibers such as is manufactured by Veratec, Inc., a Division ofInternational Paper Company, of Walpole, Mass. under the designationP-8,

The topsheet 24 may be joined to the absorbent core 28 and to thebacksheet 26 by attachment means (not shown) such as those well known inthe art, Suitable attachment means are described herein with respect tojoining the backsheet 26 to the absorbent core 28. As used herein, theterm "joined" encompasses configurations whereby an element is directlysecured to the other element by affixing the element directly to theother element, and configurations whereby the element is indirectlysecured to the other element by affixing the element to intermediatemember(s) which in turn are affixed to the other element. In a preferredembodiment of the present invention, the topsheet 24 and the backsheet26 are joined directly to each other in the diaper periphery 60 and areindirectly joined together by directly joining them to the absorbentcore 28. The topsheet 24 or backsheet 26 may be directly Joined to theabsorbent core comprising the superabsorbent foam by forming the foam onthe topsheet material or backsheet material. Alternatively, e.g., wherethe absorbent core 28 is a free superabsorbent foam sheet or afoam-bearing structure (e.g., paper tissue comprising the superabsorbentfoam), the topsheet or backsheet may be joined to the core by anyattachment means such as are known in the art.

The backsheet 26 is positioned adjacent the garment surface 100 of theabsorbent core 28. The backsheet 26 may be joined to the absorbent core28 by forming the foam on the backsheet material or, e.g., where a freesuperabsorbent polymer foam sheet or other foam-bearing structure isused as the absorbent core 28, by attachment means (not shown) such asthose well known in the art. For example, the backsheet 26 may besecured to the absorbent core 28 by a uniform continuous layer ofadhesive, a patterned layer of adhesive, or an array of separate lines,spirals, or spots of adhesive. Adhesives which have been found to besatisfactory are manufactured by H. B. Fuller Company of St. Paul, Minn.and marketed as HL-1258 or H-2031. The attachment means will preferablycomprise an open pattern network of filaments of adhesive as isdisclosed in U.S. Pat. No. 4,573,986 entitled "DisposableWaste-Containment Garment", which issued to Minetola, et al. on Mar. 4,1986, more preferably several lines of adhesive filaments swirled into aspiral pattern such as is illustrated by the apparatus and methods shownin U.S. Pat. 3,911,173 issued to Sprague, Jr. on Oct. 7, 1975; U.S. Pat.No. 4,785,996 issued to Ziecker, et al. on Nov. 22, 1978; and U.S. Pat.No. 4,842,666 issued to Werenicz on Jun. 27, 1989. Each of these patentsare incorporated herein by reference. Alternatively, the attachmentmeans may comprise heat bonds, pressure bonds, ultrasonic bonds, dynamicmechanical bonds, or any other suitable attachment means or combinationsof these attachment means as are known in the art.

The backsheet 26 is impervious to liquids (e.g., urine) and ispreferably manufactured from a thin plastic film, although otherflexible liquid impervious materials may also be used. As used herein,the term "flexible" refers to materials which are compliant and willreadily conform to the general shape and contours of the human body. Thebacksheet 26 prevents the exudates absorbed and contained in theabsorbent core 28 from wetting articles which contact the diaper 20 suchas bedsheets and undergarments. The backsheet 26 may thus comprise awoven or nonwoven material, polymeric films such as thermoplastic filmsof polyethylene or polypropylene, or composite materials such as afilm-coated nonwoven material. Preferably, the backsheet is athermoplastic film having a thickness of from about 0.012 mm (0.5 mil)to about 0.051 mm (2.0 mils). Particularly preferred materials for thebacksheet include RR8220 blown films and RR5475 cast films asmanufactured by Tredegar Industries, Inc. of Terre Haute, Ind. Thebacksheet 26 is preferably embossed and/or matte finished to provide amore clothlike appearance. Further, the backsheet 26 may permit vaporsto escape from the absorbent core 28 (i.e., breathable) while stillpreventing exudates from passing through the backsheet 26.

The absorbent core 28 is positioned between the topsheet 24 and thebacksheet 26. The absorbent core 28 is generally compressible,conformable, non-irritating to the wearer's skin, and capable ofabsorbing and retaining liquids such as urine and other certain bodyexudates. As shown in FIG. 3, the absorbent core 28 has a garmentsurface 100, a body surface 102, side edges 82, and waist edges 83. Theabsorbent core 28 may be manufactured in a wide variety of sizes andshapes (e.g., rectangular, hourglass, "T"-shaped, asymmetric, etc.). Asshown in FIG. 3, the absorbent core preferably has a modified T-shape.The absorbent core may include one or more layers of absorbent materialsand a wide variety of liquid-absorbent materials commonly used indisposable diapers and other absorbent articles. The total absorbentcapacity of the absorbent core 28 should, however, be compatible withthe design loading and the intended use of the diaper 20. Further, thesize and absorbent capacity of the absorbent core 28 may be varied toaccomodate wearers ranging from infants through adults.

The absorbent core 28 comprises the superabsorbent polymer foam of thepresent invention. The superabsorbent polymer foam can be incorporatedinto the absorbent core 28 in any of the forms described herein (e.g.,particulate, sheet form, or joined to a substrate) according to anyconventional method. For superabsorbent foam in particulate form, thefoam can be incorporated into the diaper in the same manner asconventional particulate superabsorbent or absorbent gelling materials.Alternatively, a free foam sheet, cut or formed to a desired size, maybe incorporated as the absorbent core 28 or as one or more of the layersof the absorbent core. In still another embodiment, the superabsorbentfoam is incorporated as formed on one or more of the diaper components(e.g., the topsheet 24, another absorbent core material such as a layerof tissue paper, or the backsheet 26), the diaper component serving as apermanent substrate (carrier). In a preferred embodiment, thesuperabsorbent polymer foam is formed on an absorbent carrier, merepreferably a carrier providing fast wicking properties such as tissuepaper, such that the composite absorbent material acts as the absorbentcore.

The absorbent cores of the present invention may consist solely of oneor mere (a multiplicity of) layers of the superabsorbent polymer foam orfoam-bearing carriers of the present invention; may comprise acombination of layers including the foams or foam-bearing carriers; orany other absorbent core configuration including one or more of thefoams or foam-bearing carriers. Thus, the absorbent core can alsocomprise conventional absorbent core materials. Examples of othersuitable absorbent materials are creped cellulose wadding; meltblownpolymers including coform; chemically stiffened, modified orcross-linked cellulosic fibers; tissue, including tissue wraps andtissue laminates; conventional absorbent foams; absorbent sponges;conventional superabsorbent polymers and absorbent gelling materials(e.g., particulate, including fibrous, polymers or materials); or anyequivalent material or combinations of materials.

The absorbent core preferably comprises a carrier web of fiber materialand the superabsorbent polymer foam of the present invention. Such corescan be prepared by any process or technique which provides a fibrouscarrier web and the foam. In a preferred embodiment, the absorbent coreis formed by air-laying a substantially dry mixture of fibers,densifying the resultant web if desired or necessary, and then formingthe superabsorbent foam on the web. The air-laid web will preferablycomprise substantially unbonded fibers and will preferably have amoisture content of 10% or less. The foam can be formed by printing thestabilized dispersion on the web as desired and then expanding andreacting the dispersion so as to form the superabsorbent foam on theweb. The foam can be present on the web in particular areas of the weband/or in a pattern so as to provide absorption properties designed forthe intended use of the diaper 20. For example, the web having foamformed thereon can be designed according to the absorption requirementsfor boy and girl wearers. The web having the superabsorbent foam formedthereon can then be incorporated into the diaper 20 by any conventionalmethod.

Various types of fiber material can be used in the carrier web. Any typeof fiber material which is suitable for use in conventional absorbentproducts is also suitable for use in the carrier web. Specific examplesof such fiber materials include cellulose fibers, modified cellulosefibers, rayon, polypropylene, and polyester fibers such as polyethyleneterephthalate (DACRON), hydrophilic nylon (HYDROFIL), and the like.Other fiber materials include cellulose acetate, polyvinyl acetate,polyamides (such as nylon), bicomponent fibers, tricomponent fibers,mixtures thereof, and the like. Hydrophilic fiber materials arepreferred. Examples of suitable hydrophilic fiber materials in additionto some already mentioned are hydrophilized hydrophobic fibers, such assurfactant-treated or silica-treated thermoplastic fibers derived, forexample, from polyolefins such as polyethylene or polypropylene,polyacrylics, polyamides, polystyrenes, polyurethanes and the like. Infact, hydrophilized hydrophobic fibers which are in and of themselvesnot very absorbent and which, therefore, do not provide webs ofsufficient absorbent capacity to be useful in conventional absorbentproducts, are suitable for use in the absorbent members of the presentinvention by virtue of their good wicking properties. This is because,in the structures herein, the wicking propensity of the fibers is asimportant, if not more important, than the absorbent capacity of thefiber material itself due to the high rate of fluid uptake of thesuperabsorbent polymer foams of the present invention which are includedin the core. Hydrophobic synthetic fibers can also be used, but are lesspreferred.

For reasons of availability and cost, cellulose fibers are generallypreferred for use herein as the hydrophilic fiber material of theabsorbent core. Most preferred are wood pulp fibers which are alsoreferred to as airfelt.

Other cellulosic fiber materials which may be useful in certainabsorbent cores herein are chemically stiffened cellulosic fibers.Preferred chemically stiffened cellulosic fibers are the stiffened,twisted, curled cellulosic fibers which can be produced by internallycrosslinking cellulose fibers with a crosslinking agent. Types ofstiffened, twisted, curled cellulosic fibers useful as the hydrophilicfiber material of the absorbent cores herein are described in greaterdetail in U.S. Pat. No. 4,822,453 entitled "Absorbent StructureContaining Individualized, Crosslinked Fibers Having Reduced ResidualsAnd Fibers Thereof", issued to Herron, at al. on Dec. 26, 1989; U.S.Pat. No. 4,889,597 entitled "Process For Making Wet-Laid StructuresContaining Individualized Stiffened Fibers", issued to Bourbon, et al.on Dec. 26, 1989; and U.S. Pat. No. 4,898,642 entitled "Twisted,Chemically Stiffened Cellulosic Fibers And Absorbent Structures MadeTherefrom", issued to Moore, et al. on Feb. 6, 1990. Each of thesepatents are incorporated herein by reference.

The relative amount of fiber material and superabsorbent polymer foam inthe resultant web can be most conveniently expressed in terms of aweight percentage of the absorbent core. The absorbent cores preferablycontain from about 2% to about 98%, more preferably from about 5% toabout 75%, and most preferably from about 10%, to about 60%, by weightof the absorbent core, of the superabsorbent polymer foam. Thisconcentration of the foam can be expressed in terms of a weight ratio offiber to foam. This ratio may range from about 98:2 to about 2:98. Formost absorbent cores, the optimum fiber-to-foam weight ratio is in therange of from about 95:5 to about 25:75, most preferably from about90:10 to about 40:60.

In an alternative embodiment, the diaper 20 comprises a dual-layerabsorbent core comprising an absorbent member and a sheet of thesuperabsorbent polymer foam of the present invention. Typically the foamsheet is positioned subjacent the absorbent member (i.e., between theabsorbent member and the backsheet 26).

The absorbent member serves to quickly collect and temporarily holddischarged liquids and to transport such liquids by wicking from thepoint of initial contact to other parts of the absorbent member and tothe foam sheet. The absorbent member preferably comprises a web or battof fiber materials. Various types of fiber material can be used in theabsorbent member such as the fiber materials previously describedherein. Cellulosic fibers are generally preferred for use herein, woodpulp fibers being especially preferred. The absorbent member can alsocontain specific amounts of a particulate, absorbent, polymericcomposition. The absorbent member, for example, can contain up to about50% by its weight of the polymeric composition. In the most preferredembodiments, the absorbent member contains from 0% to about 8% by itsweight of a particulate, absorbent, polymeric composition. Inalternatively preferred embodiments, the absorbent member compriseschemically stiffened cellulostc fibers as previously described.Exemplary embodiments of the absorbent member useful in the presentinvention are described in U.S. Pat. No. 4,673,402 entitled "AbsorbentArticle With Dual-Layered Cores" which issued to Paul T. Weisman, et al.on Jun. 16, 1987; and U.S. Pat. No. 4,834,735 entitled "High DensityAbsorbent Members Having Lower Density and Lower Basis WeightAcquisition Zones" issued to Miguel Alemany, et el. on May 30, 1989.These patents are hereby incorporated herein by reference. Absorbentmembers having a storage zone and an acquisition zone having a loweraverage density and a lower average basis weight per unit area than thestorage zone so that the acquisition zone may effectively andefficiently rapidly acquire discharged liquid are especially preferredfor use herein.

The absorbent member can be of any desired shape, for example,rectangular, oval, oblong, asymmetric, or hourglass-shaped. The shape ofthe absorbent member may define the general shape of the resultingdiaper 20.

The foam sheet of the present invention need not be the same size as theabsorbent member and can, in fact, have a top surface which issubstantially smaller or larger than the top surface area of theabsorbent member. The foam sheet can be smaller than the absorbentmember, for example, having a top surface area from about 0.10 to about1.0 times that of the absorbent member. More preferably, the top surfacearea of the foam sheet will be only from about 0.10 to about 0.75, andmost preferably, the top surface area of the foam sheet will be onlyfrom about 0.10 to about 0.5 times that of the absorbent member.Alternatively, the absorbent member is smaller than the foam sheet andhas a top surface area from about 0.25 to about 1.0 times, morepreferably from about 0.3 to about 0.95 times that of the foam sheet. Inthis alternative embodiment, the absorbent member preferably compriseschemically stiffened cellulosic fibers.

The foam sheet is preferably placed in a specific positionalrelationship with respect to the backsheet and/or the absorbent memberin the diaper. More particularly, the foam sheet is positioned generallytoward the front of the diaper so that the foam sheet is mosteffectively located to acquire and hold discharged liquids.

In alternatively preferred embodiments, a multiplicity of foam sheets,preferably from about two to about six foam strips or sheets, may beused. Further, additional absorbent layers, members, or structures maybe placed into the absorbent core. For example, an additional absorbentmember may be positioned between the foam sheet and the backsheet toprovide reserve capacity for the absorbent core and/or a layer todistribute liquids passing through the foam sheet to other portions ofthe absorbent core or to the foam sheet. The foam sheet may alsoalternatively be positioned over the absorbent member so as to bepositioned between the topsheet and the absorbent member.

In an alternative embodiment of the absorbent cores of the presentinvention, the absorbent core comprises a laminate (a layered absorbentcore) containing at least one, and optionally two or more, layers ofsuperabsorbent polymer foam particles of the present invention. Thelaminates preferably comprise layers or webs of fibrous materials suchas previously described, preferably a sheet of absorbent material, suchas tissue paper. Such layered absorbent structures are more fullydescribed in U.S. Pat. No. 4,578,068 entitled "Absorbent LaminateStructure" issued to Timothy A. Kramer, et al. on Mar. 25, 1986, whichpatent is incorporated herein by reference. Additional methods andapparatus for making such laminates are described in U.S. Pat. No.4,551,191 entitled "Method For Uniformly Distributing Discrete ParticlesOn A Moving Porous Web", issued to Ronald W. Kock, et al. on Nov. 5,1985, which patent is incorporated herein by reference.

The relative amount of fiber material and particulate superabsorbentpolymer foam may be the same as previously described with respect to thefoam formed on a carrier web. In addition, the foam particles may bedispersed in various weight ratios throughout different regions andthicknesses of the absorbent core. For example, the mixture of fibermaterial and the foam particles may be disposed only in certain portionsof the absorbent core. Preferably, the absorbent core contains anuniformly distributed mixture of hydrophilic fiber material and the foamparticles. The foam particles may be substantially uniformly dispersed(thoroughly dispersed) throughout the entire absorbent core as disclosedin U.S. Pat. No. 4,610,678 entitled "High-Density Absorbent Structures",issued to Weisman, et al. on Sep. 9, 1986, which patent is incorporatedherein by reference. The foam particles may alternatively be distributedin regions or zones which have higher concentrations of the foam than doother regions or zones. For example, U.S. Pat. No. 4,699,823 issued toKellenberger, et al. on Oct. 13, 1987, discloses an absorbent memberhaving a particulate, absorbent, polymeric composition distributed in apositive gradient through at least a portion of the thickness of theabsorbent member. Preferably, the concentration gradient along thethickness dimension has the lowest concentration at or near the surfaceof the absorbent member which receives liquids (i.e., the top surface)and the highest concentration at or near the back surface of theabsorbent member. This patent is incorporated herein by reference.

An alternative embodiment of the layered absorbent cores of the presentinvention is a "pouch" containing the particulate, superabsorbentpolymer foam. The pouch is a layered absorbent core as described abovewherein the number of fibrous webs equals two. The fibrous webs arejoined to each other around their periphery so as to form a large pocketin the middle of the pouch. The foam particles are encased between thefibrous webs in the pocket. Thus, the pouch is similar to a tea bag inthat the foam particles are free to swell and absorb within the pouch.The fibrous webs of the pouch preferably comprise a nonwoven material asare known in the art with the nonwoven webs being heat sealed abouttheir periphery, although other means for sealing the webs together asare known in the art, such as adhesives or ultrasonic bonds, may also beused.

The absorbent cores herein can contain a variety of optional materialsin addition to the fiber materials and the superabsorbent polymer foam.Such optional materials can include, for example, fluid distributionaids, antimicrobials, pH control agents, odor control agents, perfumes,etc. If present, these optional components will generally comprise nomore than about 30% by weight of the absorbent cores herein.

The diaper 20 preferably further comprises elasticized leg cuffs 32 forproviding improved containment of liquids and other body exudates. Eachelasticized leg cuff 32 may comprise several different embodiments forreducing the leakage of body exudates in the leg regions. (The leg cuffcan be and is sometimes also referred to as leg bands, side flaps,barrier cuffs, or elastic cuffs.) The above referenced U.S. Pat. No.3,860,003 describes a disposable diaper which provides a contractibleleg opening having a side flap and one or more elastic members toprovide an elasticized leg cuff (gasketing cuff). U.S. Pat. No.4,909,803 entitled "Disposable Absorbent Article Having ElasticizedFlaps" issued to Aziz, et al. on Mar. 20, 1990, describes a disposablediaper having "stand-up" elasticized flaps (barrier cuffs) to improvethe containment of the leg regions. U.S. Pat. No. 4,695,278 entitled"Absorbent Article Having Dual Cuffs" issued to Lawson on Sep. 22, 1987,describes a disposable diaper having dual cuffs including a gasketingcuff and a barrier cuff. While each elasticized leg cuff 32 may beconfigured so as to be similar to any of the leg bands, side flaps,barrier cuffs, or elastic cuffs described above, it is preferred thateach elasticized leg cuff 32 comprise at least an inner barrier cuffcomprising a barrier flap and a spacing elastic member such as describedin the above referenced U.S. Pat. No. 4,909,803. In a preferredembodiment, each elasticized leg cuff 32 additionally comprises anelastic gasketing cuff with one or more elastic strands, positionedoutboard of the barrier cuff such as described in the above referencedU.S. Pat. No. 4,695,278.

The diaper 20 preferably further comprises elastic waist features 34that provide improved fit and containment. The elastic waist features 34at least extend longitudinally outwardly from the waist edges 83 of theabsorbent core 28 and generally form at least a portion of the end edges64 of the diaper 20. Thus, the elastic waist features 34 comprise thatportion of the diaper at least extending from the waist edges 83 of theabsorbent core 28 to the end edges 64 of the diaper 20 and is intendedto be placed adjacent the wearer's waist. Although disposable diapersare generally constructed so as to have two elastic waist features, onepositioned in the first waist region and one positioned in the secondwaist region, diapers can be constructed with a single elastic waistfeature encircling the wearer. Further, while the elastic waist featuresor any of their constituent elements can comprise a separate elementaffixed to the diaper 20, the elastic waist features 34 are preferablyconstructed as an extension of other elements of the diaper such as thebacksheet 26 or the topsheet 24, preferably both the backsheet 26 andthe topsheet 24.

The elasticized waist features 34 may be constructed in a number ofdifferent configurations including those described in U.S. Pat. No.4,515,595 issued to Kievit, et al. on May 7, 1985 and the abovereferenced U.S. Pat. No. 5,151,092, each of these references beingincorporated herein by reference.

The diaper 20 also comprises a fastening system 36 which forms a sideclosure which maintains the first waist region 56 and the second waistregion 58 in an overlapping configuration such that lateral tensions aremaintained around the circumference of the diaper to maintain the diaperon the wearer. The fastening system may take on a number ofconfigurations such as adhesive tape tabs, mechanical closure tape tabs,fixed position fasteners, or any other means for tensioning theelasticized waist feature as are known in the art. Exemplary fasteningsystems are disclosed in U.S. Pat. No. 4,846,815 entitled "DisposableDiaper Having An Improved Fastening Device" issued to Scripps on Jul.11, 1989; U.S. Pat. No. 4,894,060 entitled "Disposable Diaper WithImproved Hook Fastener Portion" issued to Nestegard on Jan. 16, 1990;U.S. Pat. No. 4,946,527 entitled "Pressure-Sensitive Adhesive FastenerAnd Method of Making Same" issued to Battrell on Aug. 7, 1990; U.S. Pat.No. 3,848,594 entitled "Tape Fastening System for Disposable Diaper"issued to Buell on Nov. 19, 1974; U.S. Pat. No.Bl 4,662,875 entitled"Absorbent Article" issued to Hirotsu, et al. on May 5, 1987; and thehereinbefore referenced U.S. Pat. No. 5,151,092; each of which isincorporated herein by reference. In a preferred embodiment, thefastening system comprises a dual tension fastening system such asdisclosed in the hereinbefore referenced U.S. Pat. No. 5,151,092.

In a preferred embodiment, the diaper also comprises elasticized sidepanels 30 disposed in the second waist region 58. (As used herein, theterm "disposed" is used to mean that an element(s) of the diaper isformed (joined and positioned) in a particular place or position as aunitary structure with other elements of the diaper or as a separateelement joined to another element of the diaper.) The elasticized sidepanels 30 provide an elastically extensible feature that provides a morecomfortable and contouring fit by initially conformably fitting thediaper to the wearer and sustaining this fit throughout the time of wearwell past when the diaper has been loaded with exudates since theelasticized side panels allow the sides of the diaper to expand andcontract. The elasticized side panels 30 further provide more effectiveapplication of the diaper 20 since even if the diaperer pulls oneelasticized side panel 30 farther than the other during application(asymmetrically), the diaper 20 will "self-adjust" during wear. Whilethe diaper 20 of the present invention preferably has the elasticizedside panels 30 disposed in the second waist region 58; alternatively,the diaper 20 may be provided with elasticized side panels 30 disposedin the first waist region 56 or in both the first waist region 56 andthe second waist region 58.

While the elasticized side panels 30 may be constructed in a number ofconfigurations, examples of diapers with elasticized side panelspositioned in the ears (ear flaps) of the diaper are disclosed in U.S.Pat. No. 4,857,067, entitled "Disposable Diaper Having Shirred Ears"issued to Wood, et al. on Aug. 15, 1989; U.S. Pat. No. 4,381,781 issuedto Sciaraffa, et al. on May 3, 1983; U.S. Pat. No. 4,938,753 issued toVan Gompel, et al. on Jul. 3, 1990; and the hereinbefore referenced U.S.Pat. No. 5,151,092; each of which are incorporated herein by reference.Preferably, each elasticized side panel 30 comprises ear flaps and anelastic side panel member operatively associated therewith, such asdisclosed in the above referenced U.S. Pat. No. 5,151,092.

The diaper 20 is preferably applied to a wearer by positioning one ofthe waist regions, preferably the second waist region 58, under thewearer's back and drawing the remainder of the diaper between thewearer's legs so that the other waist region, preferably the first waistregion 56, is positioned across the front of the wearer. The diapererthen wraps the elasticized side panel around the wearer, generally whilegrasping at least a portion of the fastening system 36. The elasticizedside panels will typically be extended and tensioned during thisoperation so as to conform to the size and shape of the wearer. Thefastening system is secured (generally at or to the outer surface 52 ofthe diaper) to effect a side closure.

Another preferred embodiment of a unitary disposable absorbent articleof the present invention is the catamenial pad, sanitary napkin 420,shown in FIG. 4. As used herein, the term "sanitary napkin" refers to anabsorbent article which is worn by females adjacent to the pudendalregion, generally external to the urogenital region, and which isintended to absorb and contain menstrual fluids and other vaginaldischarges from the wearer's body (e.g., blood, menses, and urine).Interlabial devices which reside partially within and partially externalof the wearer's vestibule are also within the scope of this invention.As used herein, the term "pudendal" refers to the externally visiblefemale genitalia.

FIG. 4 is a Plan view of a sanitary napkin 420 embodying the presentinvention prior to it being placed in the undergarment of the wearer. Asshown in FIG. 4, a preferred sanitary napkin construction comprises aliquid pervious topsheet 424, an absorbent core 428, a liquid imperviousbacksheet 426, and a fastening system 436 for securing the sanitarynapkin to the undergarment of the wearer. While the topsheet 424, theabsorbent core 428, and the backsheet 426 may be assembled in a varietyof well-known configurations, a preferred sanitary napkin configurationis shown and described generally in U.S. Pat. No. 4,687,478 entitled"Shaped Sanitary Napkin With Flaps" issued to Van Tilburg on Aug. 18,1987, wherein the sanitary napkin 420 additionally has flaps 432 and432'. Other preferred sanitary napkin configurations are describedgenerally in U.S. Pat. No. 4,950,264 entitled "Thin, Flexible SanitaryNapkin" issued to Osborn on Aug. 21, 1990; U.S. Pat. No. 4,589,876entitled "Sanitary Napkin" issued to Van Tilburg on May 20, 1986,; U.S.Pat. No. 4,425,130 entitled "Compound Sanitary Napkin" issued toDesMarais on Jan. 10, 1984; and U.S. Pat. No. 4,321,924 entitled"Bordered Disposable Absorbent Article" issued to Ahr on Mar. 30, 1982.Each of these patents are hereby incorporated herein by reference.

Numerous other sanitary napkin embodiments are disclosed in theliterature and could provide configurations for the sanitary napkinsherein. For example, suitable configurations are described in PCTInternational Publication Nos. WO 93/01785 entitled "StretchableAbsorbent Articles," Osborn, et al.; and WO 93/01781 entitled "Curved,Shaped Absorbent Article," Johnson, et al. Both of these references werepublished on Feb. 4, 1993, and are incorporated herein by reference.Other sanitary napkin embodiments useful herein are disclosed in U.S.Pat. No. 5,009,653 issued to Osborn on Apr. 23, 1991; and U.S. Pat. No.4,917,697 issued to Osborn, III, et al. on Apr. 17, 1990. In still otheralternative embodiments, components or regions of the sanitary napkinmay be further structurally modified by folding, bending, corrugating,stacking of layers and affixing of layers to each other. Themodifications may be made by including one or more of the structuresdescribed in European Patent Application Publication Nos. 0,335,252 and0,335,253 published in the name of Buell on Oct. 4, 1989; and PCTInternational Publication No. WO 92/07535 published in the name ofVisscher, et al.

FIG. 4 shows a preferred embodiment of the sanitary napkin 420 in whichthe topsheet 424 and the backsheet 426 are co-extensive and have lengthand width dimensions generally larger than those of the absorbent core428 to form the flaps 432 and 432'. The topsheet 424 is joined with andsuperposed on the backsheet 426 to form the periphery of the sanitarynapkin 420. The sanitary napkin 420 has an inside surface 454 and anoutside surface 452. In general, the outside surface 452 extends fromone end edge 464 to the other end edge 464 and from one longitudinaledge 462 to the other longitudinal edge 462 and is the surface farthestfrom the wearer during use of the sanitary napkin. When a backsheet 426is used, it typically forms the outside surface 452. The inside surface454 is that surface opposite the outside surface 452 and in theembodiment shown is typically formed by the topsheet 424. In general,the inside surface 454 is that surface coextensive with the outsidesurface 452 and which is for the greater part in contact with the wearerwhen the sanitary napkin 420 is worn.

In the preferred embodiment of the sanitary napkin 420 as shown in FIG.4, the fastening system 436 comprises an attachment member 442comprising adhesive positioned on the outside surface 452 of thesanitary napkin 420 and a release liner (not shown) as is known in theart releasably attached to the adhesive of the attachment member 442.

Since a preferred embodiment of the sanitary napkin 420 of the presentinvention comprises flaps 432 and 432', the fastening system 436comprises flap attachment members 446 and 446' comprising adhesive onthe flaps 432 and 432' to maintain the flaps 432 and 432' in positionafter the flaps 432 and 432' have been wrapped around the edge of thecrotch portion of the undergarment. A release liner (not shown) is alsopositioned over each of the flap attachment members 446 and 446' toprotect the adhesive until the sanitary napkin 420 is used, the releaseliner being removed and the flap being wrapped around the edge of thecrotch portion of the undergarment.

The topsheet 424 may comprise any of the topsheet materials previouslydescribed in reference to the diaper embodiment of the invention. Apreferred topsheet comprises an apertured formed film. Apertured formedfilms are preferred for the topsheet because they are pervious to bodyexudates and yet non-absorbent and have a reduced tendency to allowliquids to pass back through and rewet the wearer's skin. Thus, thesurface of the formed film which is in contact with the body remainsdry, thereby reducing body soiling and creating a more comfortable feelfor the wearer. Suitable formed films are described in U.S. Pat. No.3,929,135 entitled "Absorptive Structures Having Tapered Capillaries"which issued to Thompson on Dec. 30, 1975; U.S. Pat. No. 4,324,246entitled "Disposable Absorbent Article Having A Stain ResistantTopsheet" which issued to Mullane, et al. on Apr. 13, 1982; U.S. Pat.No. 4,342,314 entitled "Resilient Plastic Web Exhibiting Fiber-LikeProperties" which issued to Radel, et al. on Aug. 3, 1982; U.S. Pat. No.4,463,045 entitled "Macroscopically Expanded Three-Dimensional PlasticWeb Exhibiting Non-Glossy Visible Surface and Cloth-Like TactileImpression" which issued to Ahr, et al. on Jul. 31, 1984; and U.S. Pat.No. 5,006,394 entitled "Multilayer Polymeric Film" which issued to Bairdon Apr. 9, 1991. Each of these patents are incorporated herein byreference. The preferred topsheet for the present invention is theformed film described in one or more of the above patents and marketedon sanitary napkins by The Procter & Gamble Company of Cincinnati, Ohioas "DRI-WEAVE."

The topsheet 424 has two sides (faces or surfaces), including abody-facing side (toward the wearer) and a garment-facing side(core-facing side). The body-facing side of the topsheet 424 generallyforms at least a portion of the inside surface 454 of the sanitarynapkin 420. In a preferred embodiment of the present invention, the bodysurface of the formed film topsheet is hydrophilic so as to help liquidto transfer through the topsheet faster than if the body surface was nothydrophilic so as to diminish the likelihood that menstrual fluid willflow off the topsheet rather than flowing into and being absorbed by theabsorbent core. In a preferred embodiment, surfactant is incorporatedinto the polymeric materials of the formed film topsheet such as isdescribed in U.S. patent application Ser. No. 07/794,745 entitled"Absorbent Article Having A Nonwoven and Apertured Film Coversheet"filed on Nov. 19, 1991 by Aziz, et al., which is incorporated herein byreference. Alternatively, the body surface of the topsheet can be madehydrophilic by treating it with a surfactant such as is described in theabove referenced U.S. Pat. No. 4,950,264 issued to Osborn.

The backsheet 426 may comprise any of the backsheet materials previouslydescribed in reference to the diaper embodiment of the invention.Preferably, the backsheet is a polyethylene film having a thickness offrom about 0.012 mm (0.5 mil) to about 0.051 mm (2.0 mils). Exemplarypolyethylene films are manufactured by Clopay Corporation of Cincinnati,Ohio, under the designation P18-0401 and by Ethyl Corporation, VisqueenDivision, of Terre Haute, Ind., under the designation XP-39385. Thebacksheet is preferably embossed and/or matte finished to provide a moreclothlike appearance. Further, the backsheet 426 may permit vapors toescape from the absorbent core 428 (i.e., breathable) while stillpreventing exudates from passing through the backsheet 426.

The absorbent core 428 is positioned between the topsheet 424 and thebacksheet 426 and comprises the superabsorbent polymer foams of thepresent invention. The foams may be incorporated into the absorbent corein the same manner as previously described in reference to the diaperembodiment of the present invention. In a preferred embodiment, theabsorbent core 428 comprises the superabsorbent polymer foam formed onan absorbent carrier, more preferably a carrier providing fast wickingproperties such as tissue paper, such that the composite absorbentmaterial acts as the absorbent core. The absorbent core preferablycomprises a carrier web of fiber material. Suitable fiber materials andcores include those described in reference to the diaper embodiment ofthe present invention. In a preferred embodiment, the absorbent core isformed by air-laying a substantially dry mixture of fibers, densifyingthe resultant web if desired or necessary, and then forming thesuperabsorbent foam on the web.

In a preferred embodiment of the present invention, an acquisition layermay be positioned between the topsheet 424 and the absorbent core 428.The acquisition layer may serve several functions including improvingwicking of exudates over and into the absorbent core 428. There areseveral reasons why the improved wicking of exudates is important,including providing a more even distribution of the exudates throughoutthe absorbent core 428 and allowing the sanitary napkin 420 to be maderelatively thin. (The wicking referred to herein may encompass thetransportation of liquids in one, two or all directions (i.e., in thex-y plane and/or in the z-direction)). The acquisition layer may becomprised of several different materials including nonwoven or wovenwebs of synthetic fibers including polyester, polypropylene, orpolyethylene; natural fibers including cotton or cellulose; blends ofsuch fibers; or any equivalent materials or combinations of materials.

In use, the sanitary napkin 420 is secured on the inside of the crotchportion of an undergarment with the pressure-sensitive adhesive fastenerside of the sanitary napkin 420 toward the crotch portion of theundergarment. Thus, the undergarment serves as the landing member forthe fastening system 436. The release liner is removed from theattachment member 442 and the sanitary napkin 420 is secured in positionby pressing the exposed pressure-sensitive adhesive fastener 442 firmlyagainst the crotch material of the undergarment. The release liner isremoved from the flap attachment members 446 and 446' to expose theadhesive. The flaps 432 and 432' are then folded under the crotchmaterial of the undergarment and secured in position by pressing theexposed adhesive fastener 446 and 446' firmly against the crotchmaterial of the undergarment, to form a double wall barrier.

TEST METHODS

In describing the present invention, a number of fluid handling,structural and mechanical characteristics of the superabsorbent foamsare set forth. In some instances, procedures for determining andmeasuring certain of these characteristics are described above or arereferenced from other patents or publications. In the remaininginstances, such characteristics can be determined and measured using thefollowing test fluids and test methods.

In each of the test methods, it is important that the foam sample to betested has essentially the same morphology of the foam sample asoriginally formed. Thus, the foam materials should be tested without anypreliminary mechanical treatment which is likely to disturb themorphology, e.g., such as sieving or grinding. Where it is necessary ordesired to use a cut piece of foam, efforts should be made to avoid edgedensification such as described in reference to flexibility testing.

1) TEST FLUIDS--SYNTHETIC URINE

The synthetic-urine utilized in the following tests is a salt solutionin distilled water with the surface tension of the solution adjusted to45 dynes/cm with about 0.0025% of an octylphenoxy polyethoxy ethanolsurfactant (Triton X-100, from Rohm and Haas Co.). Such a syntheticurine solution comprises 15 parts of 1% Triton X-100, 60 parts NaCl, 1.8parts of CaCl₂.2H₂ O, 3.6 parts of MgCl₂.6H₂ O and 6000 parts ofdistilled water.

2) DETERMINATION OF ABSORBENCY CHARACTERISTICS

A) ABSORPTIVE CAPACITY AND RATE/"TEA BAG" TEST

Absorptive Capacity and Rate can be determined by a gravimetricanalytical technique using Synthetic Urine as the fluid for whichAbsorptive Capacity and Rate of the foam is to be calculated. A sampleof superabsorbent polymer foam is placed within a tea bag, immersed inan excess of Synthetic Urine for a specified period of time, and thencentrifuged for a specific period of time. The ratio of superabsorbentpolymer foam final weight after centrifuging minus initial weight (netfluid gain) to initial weight determines the Absorptive Capacity. Ratecan be calculated from the Absorptive Capacity as a function of time.

The following procedure is conducted under standard laboratoryconditions at 23° C. (73° F.) and 50% relative humidity. Using a 6 cm×23 cm cutting die, the tea bag material is cut, folded in halflengthwise and sealed along two sides with a T-bar sealer to produce a 6cm ×6 cm tea bag square. The tea bag material utilized is a grade 1234heat sealable material, obtainable from C. H. Dexter, Division of theDexter Corp., Windsor Locks, Conn., U.S.A., or equivalent. Lowerporosity tea bag material should be used if required to retain fineparticles of foam material. 0.200 grams plus or minus 0.005 grams of thesuperabsorbent polymer foam material is weighed onto a weighing paperand transferred into the tea bag and the top (open end) of the tea bagis sealed. An empty tea bag is sealed at the top and is used as a blank.Approximately 300 milliliters of Synthetic Urine are poured into a 1,000milliliter beaker. The blank tea bag is submerged in the SyntheticUrine. The tea bag containing the superabsorbent polymer foam material(the sample tea bag) is held horizontally to distribute the materialevenly throughout the tea bag. The tea bag is laid on the surface of theSynthetic Urine. The tea bag is allowed to wet, for a period of no morethan one minute, and then is fully submerged and soaked for 60 minutes.Approximately 2 minutes after the first sample is submerged, a secondset of tea bags, prepared identically to the first set of blank andsample tea bags, is submerged and soaked for 60 minutes in the samemanner as the first set. After the prescribed soak time has elapsed, foreach set of tea bag samples, the tea bags are promptly removed (usingtongs) from the Synthetic Urine. The samples are then centrifuged asdescribed below. The centrifuge used is a Delux Dynac II Centrifuge,Fisher Model No. 05-100-26, obtainable from the Fisher Scientific Co. ofPittsburgh, Pa., or equivalent. The centrifuge should be equipped with adirect read tachometer and an electric brake. The centrifuge is furtherequipped with a cylindrical insert basket having an approximately 2.5inch (6.35 cm) high outer wall with an 8.435 inch (21.425 cm) outerdiameter, a 7.935 inch (20.155 cm) inside diameter, and 9 rows each ofapproximately 106 3/32 inch (0.238 cm) diameter circular holes equallyspaced around the circumference of the outer wall, and having a basketfloor with six 1/4 inch (0.635 cm) diameter circular drainage holesequally spaced around the circumference of the basket floor at adistance of 1/2 inch (1.27 cm) from the interior surface of the outerwall to the center of the drainage holes, or an equivalent. The basketis mounted in the centrifuge so as to rotate, as well as brake, inunison with the centrifuge. The sample tea bags are positioned in thecentrifuge basket with a folded end of the tea bag in the direction ofthe centrifuge spin to absorb the initial force. The blank tea bags areplaced to either side of the corresponding sample tea bags. The sampletea bag of the second set must be placed opposite the sample tea bag ofthe first set; and the blank tea bag of the second set opposite theblank tea bag of the first set, to balance the centrifuge. Thecentrifuge is started and allowed to ramp up quickly to a stable speedof 1,500 rpm, a timer is set for 3 minutes. After 3 minutes, thecentrifuge is turned off and the brake is applied. The first sample teabag and the first blank tea bag are removed and weighed separately. Theprocedure is repeated for the second sample tea bag and the second blanktea bag.

The absorptive capacity (AC) for each of the samples is calculated asfollows:

AC=(sample tea bag weight after centrifuge minus blank tea bag weightafter centrifuge minus superabsorbent polymer foam weight)/(drysuperabsorbent polymer foam weight).

The Absorptive Capacity value for use herein is the average absorptivecapacity of the two samples.

The Absorptive Rate (AR) can be calculated as follows:

AR=(AC for a given sample)/(time of sample submersion)

For example, 10 sets of samples and blanks are prepared andsoaked/submerged as for the Absorptive Capacity test, except that oneset of sample and blank is removed at one minute intervals and spun asfor the Absorptive Capacity test. The Absorptive Capacity and AbsorptiveRate is then calculated at various time intervals as desired, asdescribed above.

B) ABSORPTIVE CAPACITY AND RATE/BLUE DEXTRIN TEST

Absorptive capacity in terms of grams of synthetic urine absorbed pergram of superabsorbent polymer foam is determined by swelling the foamsamples in several aliquots of Synthetic Urine. The amount of suchSynthetic Urine actually absorbed by the foam is determined by aprocedure which involves use of a Synthetic Urine solution containingBlue Dextrin so that optical absorbence measurements can be used tocalculate the amount of Synthetic Urine that is not taken up by thefoam.

(a) Blue Dextrin Solution Preparation

A 0.03% Blue Dextrin (BD) solution is prepared by dissolving 0.3 partsof Blue Dextrin (Sigma D-5751) in 1000 parts of Synthetic Urine (SU)solution. The resulting solution has an absorbence of about 0.25 at itsabsorbence maximum of 617 nm.

(b) Foam Equilibration

Aliquots of the superabsorbent polymer foam to be tested are swelled in:(i) 20 parts of Synthetic Urine (SU) solution and (ii) 20 parts of BlueDextrin (BD) solution. The suspension in the Blue Dextrin (BD) solutionis prepared in duplicate. In most instances, 0.1-0.2 parts of dry foammaterial (to 20 parts SU or BD) is required to give a sufficiently highspectrophotometer reading relative to the Blue Dextrin referencesolution. One hour of equilibration at ambient temperature under gentlestir-bar stirring is sufficient for swelling equilibrium to be attained.After equilibration, a >3 ml aliquot of supernatant is separated fromeach suspension by decantation followed by centrifugation.

(c) Absorptive Capacity Determination

The optical absorbency (ABS) of each supernatant is determinedspectrophotometrically with an accuracy of 0.001 absorbence unit. TheSynthetic Urine solution is used as an ABS=0.0 reference. The absorbencyof the supernatant from the Synthetic Urine suspension without BlueDextrin should not exceed 0.01 absorbence unit; higher values indicatescattering from residual foam particles or residual additives, andfurther centrifugation is necessary. The absorbency of the Blue Dextrinsupernatants should exceed the absorbency of the Blue Dextrin referencesolution by at least 0.1 absorbence unit. Absorbency values below thisrange indicate the need to adjust the amount of superabsorbent polymerfoam used to prepare the suspension.

(d) Absorptive Capacity Calculation

The Absorptive Capacity of the superabsorbent polymer foam in gms/gm iscalculated from (i) the weight fraction of the foam in the suspensionand (ii) the ratio of the excluded volume to the total volume of thesuspension. Since Blue Dextrin is excluded from the foam due to its highmolecular weight, this ratio is related to the measured absorbencies.The following equation is used to calculate the Absorptive Capacity:##EQU1##

Absorptive Rate (AR) can be determined from the Absorptive Capacity as afunction of time in the same manner as for the above Tea Bag Test, i.e.,AR=AC/time. For example, the Absorptive Capacity can be determined bytesting a number of samples which have been swelled in Synthetic Urinefor several time periods, e.g., 1, 3, 5, and 10 minute intervals, fromwhich the Absorptive Rate can be determined. Alternatively, a continuousabsorption curve of absorptive capacity versus time of equilibration maybe obtained by using a phototrode-type detector for the SU and BDsolutions, from which the Absorptive Rate can be determined.

3) BET SURFACE AREA TO UNIT HASS

The specific surface area to unit mass (m² g) of the superabsorbentpolymer foam is determined using the Brunauer-Emmet-Teller (BET) gasadsorption method. This method involves adsorbing a monolayer of a gas(Nitrogen (N₂)) on a known mass of a superabsorbent polymer foam sampleat liquid nitrogen temperatures. The adsorbed N₂ is then desorbed byraising the temperature of the sample (thermal desorption) and detectedby a thermal conductivity detector (TCD) whose output is connected to anintegrating recorder. The peak area of the desorbed N₂ is thus known.Replicate desorption peaks are recorded for each sample, the average ofwhich is the signal area (A). After the sample analysis, the instrumentresponse (A_(cal)) is determined by injecting known amounts (V_(c)) ofNitrogen gas (99.99%+) into the system and the instrument response isrecorded via the integrating recorder. Acal is the average of theseveral instrument responses obtained upon injecting the known amountsof N₂. The A, A_(cal), and V_(c) values are then used to determine thespecific surface area of the sample using the multi-point BETcalculation.

The specific equipment used for these analyses is obtainable from theQuantachrome Corporation (Syosset, N.Y.) and consists of the QuantectorOutgassing Station, the Flow Controller, and the Quantasorb Jr. SampleAnalysis Unit. These instruments are used as described in the operatingmanuals for the Quantasorb Junior® Sorption System, 2/1985, incorporatedherein by reference. Various specific N₂ -Helium mixtures obtained bymixing pure N₂ and pure helium via the Flow Controller are used as theadsorbate gas.

0.75 grams ±0.05 grams of foam sample is weighed into the glass samplecell (about 2.5 ml) of the apparatus. The glass cell containing thesample is then placed into the gas flow of the instrument. The samplesare outgassed with a 30 ml/min Helium flow using the Quantector for atime sufficient to remove any gases other than Helium from the sample,typically a minimum of 4 hours. After outgassing, the gas flow ischanged to a specific N₂ -Helium gas mixture. The glass sample cell isimmersed in liquid Nitrogen and allowed to reach equilibrium. Anadsorption curve is generated. The adsorbed N₂ is then desorbed byremoving the liquid Nitrogen and immersing the glass vial in warm tapwater. The adsorbed N₂ generates a desorption curve and a peak value(used to calculate the signal area (A)). Adsorption/desorptionmeasurements are performed on each sample using different N₂ -Helium gasmixtures.

The specific surface area S_(g) is calculated as follows:

    S.sub.g =S.sub.t /W;

wherein W is the weight of the sample and S_(t) equals X_(m)(6.02×10²³)A_(cs) ;

wherein A_(cs) is the adsorbate cross sectional area. For N₂, S_(t)becomes X_(m) (3.483×10³)m² ; wherein X_(m) equals 1/(S+I). S is theslope and I is the Y-intercept of the plot of 1/X[(P_(o) /P)-1] versusP/P_(o).

In calculating the x and y values for the above plot, X equals (A)X_(c)/(A)_(cal). A is the signal area; A_(cal) is the calibration area; andX_(c) equals P_(a) M_(a) V_(c) /6.235×10⁴ T. P_(a) is the ambientpressure; M_(a) is the molecular weight of the adsorbate which for N₂becomes 28.0134; V_(c) is the calibration volume; and T is the ambienttemperature in ^(o) K. P is the partial pressure of the absorbate; andP_(o), the saturated vapor pressure, equals P_(g) +P_(a) ; wherein P_(g)is the vapor pressure (above ambient) and P_(a) is the ambient pressure.

4) DETERMINATION OF FLEXIBILITY

Foam flexibility can be quantified by referencing a test procedure whichis a modification of the ASTM D 3574-86 test used to determineflexibility of cellular organic polymeric foam products. Such a modifiedtest utilizes a foam sample which is 7 cm ×0.8 cm ×0.8 cm and which hasbeen saturated to its absorbent capacity (i.e., soaked in SyntheticUrine for about 15 minutes) with Synthetic Urine at 37° C. The SyntheticUrine-saturated foam strip is bent around a 0.8 cm diameter cylindricalmandrel at a uniform rate of 1 lap in 5 seconds until the ends of thestrip meet. The foam is considered flexible if it does not tear or breakduring this test, i.e., if it passes one bending cycle.

It is important that the cutting process used to make the foam samplesdoes not introduce edge defects in the foam strip. The foam strips ofthe requisite size should be cut from larger blocks of the same materialusing a sharp reciprocating knife saw. Use of this or an equivalent typeof sharp cutting device serves to substantially eliminate sample edgeflaws and edge densification effects which could have adverse impact onthe accuracy of certain of the measurements made in carrying out thetest procedure. In addition, caliper or thickness measurements should bemade when the absorbent structure sample is under a confining pressureof 0.05 psi (350 Pa).

5) EXTRACTABLE LEVELS DETERMINATION (I.E., INSOLUBLES CONTENT)

The solubility/insolubility of a particular component, material, etc.(hereinafter "sample") in the solvent is determined by determining theextractability of the component or material in the solvent at ambienttemperature (about 22° C.). Extractability is determined by agravimetric procedure wherein the sample to be tested is mixed with thesolvent, the mixture is filtered, and the weight of the resultantresidue is determined.

Into a 500 ml Erlenmeyer flask is weighed accurately (to ±0.1 mg) about0.25 grams of the sample to be tested (Ws). 250 ml of the solvent isadded, and the mixture is stirred slowly for 1 hour. After this hour haspassed, stirring is stopped and the mixture is filtered using anErlenmeyer flask fitted with a pre-weighed 0.45 micron filter paper. Thetare weight of the filter is (Wp). After filtration, the filter paperwith residue is removed from the funnel and dried in a 120° C. oven for1 hour. The filter paper is cooled and reweighed to obtain (Wr).(Wr)-(Wp) equals the weight of the residue (r).

Along with the sample, a second flask containing 250 ml of the solventis stirred and filtered and the filter paper is dried as for the samplein order to obtain a blank weight (Wb). The blank filter paper ispre-weighed (Wbf). Wb-Wbf equals the blank residue (br). The adjustedsample residue (ar) equals (r)-(br).

The level of extractables equals [(ar)/(Ws)]×100.

EXAMPLE I

A superabsorbent polymer foam may be formed as follows: Approximately250 grams of aqueous (50% by weight) acrylic acid are 75% neutralized tosodium acrylate using 0.1N NaOH. To avoid the formation of polyacrylicacid, the neutralization is performed by gradually adding the NaOH tothe acrylic acid solution with gentle mixing and cooling (with dry ice).

A reaction mixture is prepared as follows:

200 grams of the above prepared aqueous sodium acrylate solution; 0.50grams N,N'-methylbisacrylamide; 1.3 grams V-50(2,2'-azobis(2-amidinopropane) dihydrochloride, available from WakoChemicals USA, Inc.); and 20 grams PEG-600 (polyethylene glycol having aweight average molecular weight of about 600, available from the UnionCarbide Co. of Danbury, Conn.) are added to a 1 liter glass reactorfitted with temperature and pressure controls and a high shear mixingapparatus (e.g., an "Ultra Turray" mixer, available from the TekmarCompany of Cincinnati, Ohio). The reactor is at ambient temperature(about 22° C.) and pressure (about 1 atm). A mixture of 60 grams ofFreon 1,1,2 (1,1,2-trichlorotrifluoroethane, available from AldrichChemical of Milwaukee, Wis.); 3.5 grams SPAN®20 (sorbitan monolaurate,available from Aldrich Chemical); and 6.5 grams TWEEN®20 (ethoxylatedsorbitan monolaurate, available from Aldrich Chemical) is then added tothe reactor. This latter mixture is prepared in advance by adding thecomponents to a reactor similar to the one described above and mixingwell.

The reaction mixture is mixed at about 850 rpm for about 10 minutes tostably disperse the Freon 1,1,2. The mixing apparatus is then removedfrom the dispersion. A foam product is formed by increasing the reactortemperature to 60° C. and maintaining the temperature at 60° C. forabout 1 hour; followed by increasing the temperature to 80° C. andmaintaining it at 80° C. for about 30 minutes; followed by increasingthe temperature to 120° C. and maintaining it at 120° C. for about 30minutes. The reactor is then cooled to about ambient temperature (about22° C.).

A mixture of 5 grams of glycerol and 25 grams of isopropyl alcohol isadded to the foam in the reactor to impregnate the foam with themixture. The temperature of the reactor is then increased to andmaintained at 180° C. for about 1 hour. The reactor is cooled to ambienttemperature (about 22° C.), after which the foam is removed from thereactor and openly placed in a humidified room (80% relative humidity)for 6 hours.

An open-celled superabsorbent polymer foam so formed would have thefollowing properties: a gel volume (i.e., absorptive capacity) of 50g/g; a swelling rate (i.e., absorptive rate) of 2.8 g/g/sec; an averagecell size of 30 microns with a standard deviation of ±16 microns; and asurface area to mass ratio of 0.48 m² /g.

While particular embodiments of the present invention have beenillustrated and described, it would be obvious to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope of this invention.

What is claimed is:
 1. An absorbent member comprising:(i) a wicking web;and (ii) a superabsorbent polymer foam disposed on said wicking web;said superabsorbent polymer foam comprising a plurality of mutuallyconnected struts of superabsorbent polymer material to form open cells,said foam having: (a) a surface area to mass ratio of at least about 0.2m² /g; (b) an average cell size of less than about 100 microns; and (c)a density of less than about 0.5 g/cm³.
 2. The absorbent member of claim1 wherein said superabsorbent polymer foam has an average cell size ofless than about 50 microns.
 3. The absorbent member of claim 2 whereinthe superabsorbent polymer foam has a cell distribution value of fromabout 1 to about
 3. 4. The absorbent member of claim 1 wherein saidsuperabsorbent polymer material of said foam comprises reactants capableof forming said superabsorbent polymer material, said reactants beingcapable of being made substantially soluble in a solvent; said reactantsbeing expanded in the presence of (i) said solvent and (ii) a blowingagent which is substantially insoluble in said solvent to form anexpanded structure; said reactants being reacted to form asuperabsorbent polymer material which is substantially insoluble in saidsolvent.
 5. The absorbent member of claim 4 wherein said reactantscomprise:(a) a substantially water-soluble, unsaturated monomercomprising neutralized carboxyl groups; and (b) a substantiallywater-soluble internal crosslinking agent which is capable of reactingwith said monomer to form a substantially water-insoluble polymermaterial; said internal crosslinking agent being selected from the groupconsisting of compounds having at least two polymerizable double bondscompounds having at least one polymerizable double bond and at least onefunctional group reactive with said monomer, compounds having at leasttwo functional groups reactive with said monomer, polyvalent metalcompounds which can form ionic linkages, and mixtures thereof.
 6. Theabsorbent member of claim 5 wherein said internal crosslinking agent isexpanded and reacted with said monomer in the presence of a surfactant,an initiator, a viscosity control agent, or mixtures thereof.
 7. Theabsorbent member of claim 6 wherein the superabsorbent polymer foam isflexibilized by a plasticizer.
 8. The absorbent member of claim 6wherein said superabsorbent polymer material additionally comprises anexternal crosslinking agent.
 9. The absorbent member of claim 6wherein(a) said monomer is selected from the group consisting ofpartially neutralized acrylic acid, maleic acid, methacrylic acid,fumaric acid, itaconic acid, maleic anhydride, ethylacrylate,butylacrylate, and mixtures thereof; (b) said internal crosslinkingagent is selected from the group consisting ofN,N'-methylenebisacrylamide, triallylamine, triallylphosphate, and di-orpoly-glycidyl ethers of aliphatic polyvalent alcohols; (c) said solventis selected from the group consisting of water, lower alcohols, ormixtures thereof; and (d) said blowing agent comprises a substantiallywater-insoluble compound having a vaporization temperature of less thanabout 50° C.
 10. The absorbent member of claim 1 wherein saidsuperabsorbent polymer foam is formed on said wicking web.
 11. Theabsorbent member of claim 10 wherein said superabsorbent polymer foam isformed on said wicking web in a discontinuous pattern.
 12. The absorbentmember of claim 11 wherein said wicking web comprises cellulosic fibers.